Reef Builders - The Source for Reef Keeping Information

I have had two Ricordia just disappear overnight! The first one was a week or so ago which was about the size of a quarter. I had it at the front bottom of the aquarium and the next morning it was gone. I looked everywhere but can't find it. Then a few days ago I had this beautiful Lime green Ricordia Yuma one that I paid 80.00 for and it was about the size of a half dollar. I had glued it to a large rock and it some how came off so I placed it at the bottom thinking it may not like as much light and the next morning I got up and it was gone! This really has me puzzled especially since I have another beautiful one even more expensive and I don't want to loose it to. Any suggestions as to what could be happening? Everything else in the tank is doing good and my test are all fine with the exception of DKH running a little high. I have also done my water changes weekly and test constantly . I have several other quarter size Ricordia which have stayed at the bottom for several weeks that are still there. Any ideas?

Nancy