I'd like to help you, but you didn't provide enough information.
From what I understood - the night ends after you undress her, the night can end in 4 ways:
- time runs out
- you get caught by either Yuna or Kan
- your camera battery runs out (stamina)
- you end the night early by clicking "end" button next to the timer
If you're sure, you aren't getting caught by Yuna, your battery is full and there's plenty of time left, then it must be due to you accidentally clicking "end" button next to the timer.
If you have any more info on the problem, feel free to reply, we'll try to figure something out.
Cheers