None of your tank specs raise any red flags for me ... our tanks are similar. Shrimp are sensitive to nitrates and nitrate testing kits are notorious for being innaccurate. The one test I did not see in your list is nitrites -- high levels can be very deadly to fish at least. Shrimp are more sensitive than fish, so if anything is wrong, they'll often be the first to let you know by dying.

I recently killed my shrimp (amano and cherry) off by treating ich with a copper compound and then adding another chemical that broke up the compound leaving the toxic copper by itself -- shrimp were all dead the next morning. All seems OK now as my new shrimp are doing fine. Good rule of thumb: stay away from copper if you have invertebrates.