My 8 month old eats rat weanlings. The prey should be about as big as the widest part of your snake's body.

If it makes you feel any better, it usually takes my BP 10-15 minutes to find her rat's head and start eating after she "kills" it, but she figures it out eventually. As long as your snake is eating, don't worry too much about it. Maybe some day she'll learn how to snake, maybe not