I have an entirely baseless theory, which like all good baseless theories, only stem from a number of obscure facts. Consider the following.

-Ichigo's monsterous spirutual power has been attributed to fact that he is a "pure-blooded Shinigami." (I haven't found this said exactly in the Manga, but it was present in a "canon" arc. I may just not be looking in the right place.)

Part of this points that Masaki was also a Shinigami. You don't get a pureblood by mixing a pure and a pure breed of, for all intents and purposes, species (It has been proven exhaustively that there are differences between different kinds of souls, and is reasonable to assume that human souls and Shinigami souls are effectively two different kinds.)

Through this I warrant that Karin and Yuzu are simply not old enough to generate high levels of reiatsu. Uryuu's battle with Kurotsuchi Mayuri in Soul Society proved the body has a natural ability to impede on forming and gathering reiatsu. I reason that because Ichigo is essentially physically mature(at age 15, the main goal of puberty- reproductive ability, is already achieved, most changes after this are 'cosmetic' in the form of hair growth, voice change, and maybe gaining a few more inches in height.)

-Isshin refers to the Vaizard group as "The criminal group of Ex-Shinigami that uses forbidden spells... to acquire the power of hollows." and then subsequently remarks "We never found their base of operations, nor uncovered their goals."

Thanks to the TBTP mini-arc, we know that they are not criminals, but merely victims of Aizen Sousuke's machinations. From the evidence we can assume that Urahara Kisuke was kicked out/fled due to this incident.

Perhaps it's just me and my baseless theory, but Isshin doesn't know the truth, while Urahara does. Doesn't this strike you as the intsiest bit curious? While it's entirely possible that the TBTP arc was a Retcon, my baseless theory and I have our doubts.

-Isshin's loss of his Shinigami powers is never explained in the short, short instant the truth is revealed. There have been many speculations as to why he lost his Shinigami power, the common one being his Gigai.

During the beginning of the series, Rukia states that a Shinigami enters a Gigai to recover their spiritual power. Now, we know that Rukia's Gigai was rigged to turn her into an ordinary human soul, and to implant the Hougyoku in it's depths to hide it. Although, we also know that Urahara created a Gigai that couldn't be tracked by Soul Society.

If we assume this Gigai suppresses reiatsu, it's a possible explanation for Isshin's loss of power, and possibly reclaiming them after 20 years. The only explanation I can create for this one is that after his reiatsu was suppressed to the point of removal, Isshin's soul "sycronized" with the Gigai, which allowed him to slowly regenerate his powers over the time.

-There are seperate "military" powers in Soul Society that are not explicitly stated as a part of the Gotei 13 squads. Right now the only supporting factor are the Kido Corps, since the Correction's squads are traditionally lead by the 2nd Division Captain, in past and present cases: Shihouin Yoruichi and Soi Fon respectively.

-Seireitei is for all intents and purposes, a military base. Just because the enemy happens to be corrupted souls doesn't mean they're not militaristic about doing it. They have an R&D section, they have heavy artillery, and ruling members of Soul Society all reside there (The Noble Houses, for example, the major exception being the King of Soul Society, who resides in a seperate dimension along with Squad Zero.)

Here is the meat and mettle of my theory:

It's not unreasonable to assume that Seireitei would have sent out a squad or advance party in search of the Vaizard. Using Modified Souls as an example, it's not unreasonable to assume that Soul Society and the Central 46 have a very specific method of dealing with freak accidents: Get rid of them, and bury the everliving hell out of them. While that may be the Corrections divisions territory, it's also reasonable to assume that they would be cleaning up Soul Society's side of the inicident, while a second squad is dispatched to find things out in the Real World.

For the sake of not stepping on any more toes, I'll leave out how things might go about and get to the point.

Urahara, Isshin, and Ryuken all know about eachother. Ryuken probably kept his distance from Urahara, given he's an ex-Shinigami, Isshin and Masaki, for whatever reasons, gave up being a part of this expeditionary force, and got Reiatsu-conceiling Gigai and started raising a family. We can assume around this time, Isshin and Masaki lost their spiritual power, thus marking the 20 year period defined by Isshin.

We can assume at this point, Ichigo has been born(perhaps Ichigo was the loss of their powers?). Yuzu and Karin are born sometime within the 5-6 years prior to Masaki's death. (I can't remember if Ichigo was 4 or 5 when Masaki died, it's 7:30 in the morning and I've been up all night.)

My theory in the short and sweet format:

-Isshin and Masaki were apart of an Expeditionary force to find the Vizard.
-For some reason, they gave up, and started a family.
-Urahara and Isshin have had buisiness, given their shared knowledge and familiarity.

Like all good baseless theories, mine is probably wrong, but if you've read this far, it was atleast entertaining, right?

A couple ending oppinion's I'd like to put out there:

Kotetsu Isane and Kurotsuchi Nemu are the sexiest of the Bleach Women, with honorable mentions to Captain Unohana Retsu. Captain Unohana for the simple fact people would rather be obliterated by Genryuusai's Ryuuji Jakka Bankai than get on her bad side.

^ sex = reiatsu draining activity
few years of no sex = reiatsu build up

WTH?! if isshin use that to explain y he lost his power.. i will drop the manga and the anime at that moment! 

what would make soifon bankai effective this time, after all it does not gives any effect on baragan in the first time..

thank you!

i like soifon canon ball
hachigen cast that box thingy works somewhat like an amplifier dmg
imagine ichigo hollow cast the black cero(the cero he cast on Ulqoirra) with aid of that box..
woot! 

stepping up next to meet their makers i guess its Hallibel 

If the logic is still exist in the Bleach Universe, the way to beat Stark is just letting him fire those ceros until he runs out of ammo. Still Bullets' trajectory is a straight line. As long as you focus on the gun's line of sight, you can dodge the bullet.

Just because stark uses guns doesn't mean he's cool. At least not to me. The only techniques i see he was using are ceros and dodging. That's all.

Cool lines? It was all but conceited lines.

huhuh when la nel adult want to help?

alucard is better. while dante must be cool