Sadly you were provided with erroneous info, pinkies are indeed way too small for any BP (hatchlings are usually started with hopper moving quickly to adult mice) which could be one of your problem.

If the prey is too small the animal will not always recognize it as a prey and if it does because of the small size it will likely not constrict it.

Lack of constriction and eating backward are not something you need to worry about.

Simply offer your BP and appropriate size prey, something equal to you BP’s girth size (widest part of your BP’s body) once every 5 to 7 days.
Good luck.