What type of lighting are you using? If it's under LEDS could be getting scortched. A little more info on the setup and parameters would be helpful. Any alk or parameter swings lately?
On the deeding side of things, if you have some mysis feed it that, give it something a little more substantial, they'll devour mysis like crazy. In my experience plates are pretty resilient and hardy, especially when you're constantly feeding them.