Q,
-how thick is your ADA soil ?
-Did you add in any powder/liquid bacteria to aid in tank cycle?
-and did you add in any new plant/moss few days or during the time you put in the shrimp?
-Have you done few large water change prior on getting the shrimp?
-Your shrimp swim/dash fast all over your tank before drop down and died? or they keep themselves under rock/plant all the time?
-How high is your tank temperature?

Few reasons why your shrimp died,
-Tank not fully cycle, 2 mth may not be enough to fully cycle a ADA soil based tank esp without adding any bacteria additive or provide optimum
environment to speed up bacteria colonization.
-DIY co2 , if you run it for 24hrs, and have too much of it, shrimp may died of suffocation and also swing ph too low. Bacteria in filter will died off if ph go lower then 5.0.
-New Plant or plant/moss still have fert/pesticide on them (never induce any new plant/moss into shrimp without testing it first with one shrimp in a bucket) unless you got it from your friend healthy shrimp tank.
-Try use a long thin stick poke the soil to release air bubble if you haven't done that, this will help release trapped ammonia in the soil.
-Adding fert that not shrimp safe or adding stuff that contain copper.
-Anyone spray insecticide nearby your tank? like less than 10 feet away?
-Never wash properly your hand (got shampoo/oil residue on your hand when handling your tank)?

i suggest you go get a standalone nitrite NO2 test kit(cost around rm20-30++) to test your tank, i suspect it still at stage 2 cycle(lot of NO2) which are highly poisonous. If none N02 is detected then your tank is at stage 3 where Nitrate(NO3) is building up and probably cycled, just need a large 70-80% water change to bring nitrate under control.  However if NO2 is present then you need to do NO2 test every 3 days till it become zero, no live stock(esp shrimp should be added).
Fish is more hardy then shrimp when dealing with tank cycle , some will survive the ordeal but with permanent injuries and their live span shorten.
Some can't stand the cycle will jump out or died.