My guess would be that you don't have a good "routine" down with your snake yet.

For example, on Monday afternoons/evenings just about every single one of my snakes will eat without a hitch ... but if I were to try and feed just one or two (even if they are normally great eaters) on a Thursday morning, I would definitely have some problems because they are just not used to the difference in routine.

Routine, routine, routine!! ... That is the secret to feeding ball pythons ... Pick one method, on the same day each week, and do EVERYTHING the same each time ... before you know it, your ball python will be telling you when it's time to eat.

-adam