First they all grow at different rate, there are many factor to take in consideration which is why you should not compare your BP to another or think there is something wrong because someone as a bigger BP even though it hatched at the same date (it depends on weight out of the egg, how fast they got started, feeding frequency, prey size etc)

If you have feeding issue I would not suggest to take your animal to a pet store (I would not jeopardize one of my snake by doing that if I were you)

If you have not done so already get a 15 quarts tub with 2 tight and identical hides

Something like this

Provide 80-84 on the cool side and 90-94 on the warm side with 50% humidity

Once moved DO NOT handle your BP once a week has passed try to offer a live rat pup (pre-scent the room and put the rat pup in your snake enclosure)

If you have a refusal wait a week before offering again.

Also get a cheap kitchen scale to monitor your BP's weight.