M.R.
I had to have NSTs with all 3 of pregnancies from 22-26 onwards. I agree with the full tummy. Baby tended to move more after I ate a good meal. I also found scheduling first thing in the morning was the best time for me personally because that was always my babies most active time apart for at night when I would lay down to sleep.
A few times when my babies would not co-operate and sleep the whole time the doctor would use a buzzer on my belly to wake him up. I personally thought it was a little mean to the little guys but no matter what I did they just kept sleeping.
Good luck. It is a big chunk of time that you get to just lay still and enjoy listening to your little peanut.