Do push ups on your knuckles
are you punching properly? the punch should land on the heavy bag with your four knuckles in a straight horizontal line, you may have your fist slanted when it lands, also, you could be starting out punching it too hard, you should built up to the power punches.
Weak wrists.do some pushups and lift weights.eat raw eggs.Use a sponge gripper as well.
you need to get your wrist stronger so simple
the gym will be helpful
I always hurt my wrist when i punch. Does anyone know how i can stop that ?