Sorry for your losses. I have nothing to offer in terms of a cause or solution.
Gestation/incubation times are never exact in any animal. I can never understand the way we as a hobby approach 'cutting' the way we do. It's like kids who don't want to wait for Christmas to open their presents, but there's no one to stop them or guilt them into self control. Reptiles have been crawling out of eggs on their own for an amount of time so great it is difficult to comprehend. I can understand once the first snake naturally pips if you want to assist the process for the rest, but to just cut them open after an average number of days seems so unscientific and risky for an amateur hobbyist that I don't intend to practice it myself.
I can totally understand learning to read the signs of dimpling, etc. but to just have a number set in stone seems a foolish practice in my humble opinion.