Nitrates alone rarely kill shrimp, even caridina. I would guess kH is your issue, give it a good check with a test kit and maybe consider adding salty shrimp gH kH+ with RO water. Blue bolts are a notoriously frail breed of shrimp that require a lot of care. Also, lower the frequency and volume that you do water changes! It could also be a stressor, I would say it’s a good idea to just top off water levels as needed especially if using salty shrimp and RO. You could get water changes down to like once a month if not longer with that strategy!
thesparks3 
Have the shrimps been molting?
Nitrates alone rarely kill shrimp, even caridina. I would guess kH is your issue, give it a good check with a test kit and maybe consider adding salty shrimp gH kH+ with RO water. Blue bolts are a notoriously frail breed of shrimp that require a lot of care. Also, lower the frequency and volume that you do water changes! It could also be a stressor, I would say it’s a good idea to just top off water levels as needed especially if using salty shrimp and RO. You could get water changes down to like once a month if not longer with that strategy!
The tank is neither cycled nor mature. Three weeks should have been three months.