Marsha, I don't know what your schedule looks like but you may want to slow it down and maybe not hold it so long at the process temp. Your glass seems to be pooling at the bottom and the long hold may be pulling the rest of the glass through the hole. Making sure that your blank is at least 2" larger than the hole will help also.
I'll look at my schedule and give that a try... I use two layers of glass and they fit the outside of the ring.