It's very hard to diagnose from afar.
Some general considerations. Check for broken equipment, for example, cracked heater or magnet casings.
Are you calibrating your refractometer? Check out Randy-Holmes Farley's articles on DIY calibration fluid.
Are you doing weekly/biweekly water changes? It helps mitigate problems in water chemistry which we are mostly unaware of.
While it's a little expensive it might be worthwhile in the beginning to consider an ATI or a Triton water test. Test all of your parameters at the same time you send off your sample this can give you a check on the accuracy of your testing. I think ATI throws in a few extra tests for free such as a check of your RODI water.
You don't mention anything about lighting but if you haven't watch some of the BRS videos on lighting. Probably not the case for the corals you are keeping but it's worth mentioning. I think they give some excellent primers. I know in my case in the beginning I had my lights way to close to the tank. I didn't realize the importance of spread. I thought corals were more like house plants. It took me a while to get it. I'm kind of thick.