Are you removing the frogs from the tank when you are feeding them individually? That might make the one feel a little stressed out. If not, maybe try doing so.
I would try and leave some flies in with them after feedings, it might feel more comfortable or just want to eat on a funky schedule.
I have a newly morphed Gray, lost its tail late last week and it generally eats 4-5 flies in the morning.
But it also eats a few here and there at night when its running around and most active. Maybe try an individual night feeding for that one.
I hope you find something that works!