HELP!!! LT Plate Is Receding

[The Grizz]

I have had this Long Tentacale Plate since January 9 this year, Recently I have noticed that it is starting to recede around the bottom & on 2 small spot's across from each other. I had it on the sand bed but read some where that was not a good thing, so I put a 1" thick flat rock under it. I also moved the flow so it was not waving around so much. Also it does not seem to want to take any PE Mysis at all, even when I first got it. Parameter's are all good.