ive seen a FT feeding video from a big breeder, some BPs constricted the food. He used tweezers to feed, and when a snake wanted to just swallow it he wiggled the food around and pulled a bit and didnt let go, and that triggered many of the lazy snakes to properly constrict it.

you can try that, as soon as the snake shows serious interest you make it a bit difficult by wiggling the food around a bit, simulating some resistance.