I'd do a larger water change to start with. In cases like these I can do an 80 - 90% water change and get away with it. Heavily planted tanks tend to weather large water changes better, in my experience.
Checking the GH and KH couldn't hurt. Shrimp don't require much calcium - far less than snails, so that's probably not the issue, but it's possible. My neocaridina are kept in tanks with GH anywhere from 3 to 8 degrees. I also keep my tanks at a KH of 3 to 9 degrees. RCS don't seem to care how high the KH gets, in my experience, but I'd keep the GH below 8 as I've had problems with them in tanks where the GH is boosted too high (for platies in my case).
Do you treat the water when you do changes? I assume so, but if not, adding some prime couldn't hurt - that'll take sanitizers and heavy metals out of the equation.
I've had issues with RCS in dirted tanks in the past, but those were the result of PH crashes for the most part. How big were the shrimp when you added them to your tank? Larger shrimp don't acclimate well. It could just be they've been headed downhill since you added them due to acclimation shock. I've lost a ton of shrimp due to acclimation in the past - smaller is better for sure.