Because you keep BPs. They like to keep us on our toes that way.
Anyway, sorry to hear about your snakes being little picky buggers. Maybe you could try warming the head up a little more with a heat lamp/hair dryer before offering if you haven't already. As to why they would refuse randomly, I can't tell you since you seem to have perfect conditions. My guess is just BP antics.
Good luck with the little guys!