The more information you give, the more likely DSHannah will be able to help you.
What server are you on?
Does it happen with all three bosses? or only one?
If only one, which one?
So when you complete Stage 1 of the boss does it go back and show that stage as Incomplete with tries taken away from your total?
Or does it just go back to Stage 1 as if you hadn't hit it at all?
Or does it show as complete but go back to Stage 1 (instead of Stage 2) the next day
As was suggested above, screenshots would probably be of great help.