Renewing kiln wash
Any time you have kiln wash sticking to the bottom of a piece is an indication that you have fired once too often on that application of kiln wash. It certainly says that you now need to clean the shelf and put a new application of kiln wash on it. (Actually, that you should have done it before this firing.) Experience will show you when you need to redo the kiln wash.
My experience shows me that i need to renew the kiln wash every time I go to tack fuse or higher.