Well the only other thing I can think of is the temp or humidity was off and slowed them down.
I would keep a eye on them and as ling as they are growing and moving leave them. Once they fill the egg up and it looks packed in their up the humidity. If you have already done that it will not hurt to keep it at that leave.
( as long as it is not to high)
It just ended up in one of those hurry up and wait hatches.
Best of luck and I hope it all works out for you!