I have experienced similar problems. The books state that the best
way is to soak the flask in water, then slowly heat it back up to
casting temperature. I have done this and it works. When I did not
have time to soak the flask, I have just reinserted it into the oven
(from room temperature or whatever) and reheated it, which also
worked perfectly! Personally, I would just left the flask in the
oven. I suppose the size of the flask in question, and the complexity
of the pattern in the flask, might have a bearing on this issue. If
it is a small flask, I would just reheat it, but not too suddenly
(not putting it straight into a 1000 degree oven).
Unfortunately, I can not reply directly to you because Orchid does
not allow this.