1). If u r burning out more than 1 ring at a time. Burn out this problem metal by it self you could have a cold spot in the burn out oven. Or have just overloaded the ovens ability .
2) if already burning out by its self increase the burn out temp by 100 degrees F. I have found this to help many labs
3) check your thickness measuring gauge making sure it still measures correctly, the wax pattern may have been to thin.
4) double check the metal is still the exact same
5). Go over all instructions making sure your doing it by the book Sometimes after doing something for a long time. By mistake something gets changed.
I hope I have helped :)