What keyboard you are using?
BWU ?
Long time no see our mkb thread legendary Abang Taobao...
Hope human10 can surface and help. Miss your post. 

Depends on how long you use and the keycaps. Try taking off the keycap and pressing.

If got difference means that the keys are slightly worn out already lo.

first.... what keyboard you using first.

check keycaps
and try press every key one by one.
perhaps your fingers force varies thats why the sound is different.

Mine ducky zero, the "pg down" button very obvious different sound n different tactile feedback

Mine ducky zero, the "pg down" button very obvious different sound n different tactile feedback

After remove the keycaps, do the silent keys you mentioned just now have clicky sounds like other normal mx blue?

Going by the number of results on Geekhack alone if you do a quick Google search for "Cherry MX Blue not clicking" or something of the sort (here and here for example), I'd wager that it's a somewhat common problem. Saw some people suggesting that a quick blast of compressed air might help, maybe that's something you can try?

For what it's worth, the Esc key on my Poker 2 is very obviously a lot less clicky than the other switches; if it were a more high-traffic key I'd probably be a wee bit annoyed but, as it is, I don't mind too much.

Lemau - not crispy due to exposure to air. 
I understand, normally if using mx blue with abs keycaps do have crispy clicky sound.
Sounds like a quality issue of assembly if you feel the clicky sounds are distinctive. 
If layman term, then is bad quality.

Should RMA. Clicky sound is the hallmark of MX Blue.

Also, good night.