The good eater: is being fed live or f/t? Big difference, if live, adult mice are far more dangerous in terms of fighting back. Hopefully you feed f/t- & some BPs
can be very picky eaters...

Are the larger mice from a different source? (maybe fed differently & so they smell different?)
The new one: have you been handling? (if so, stop...eating is "job #1") Often it's best not to even offer food to a new snake for the first 2 weeks, though most
of us are eager to verify that they do eat, & also to make them welcome. Don't offer too often...it adds stress...no more than once a week, & make sure you're
following best practices (feed in evening, dim light, not too much motion if offered by tongs, *thawed correctly so it's not spoiled*, warmed so they can target
correctly using their heat sensing pits, etc) *thawing* is best in cool water until soft thru-out (feel by hand), only then immerse in very warm water to bring it
up to more lifelike warmth, & most ppl have good luck using a blow-dryer to warm especially the head, immediately before offering. IF you've been thawing in
warm water, or by leaving it set out on the counter, understand that spoilage occurs that your snake can smell long before you do, & BPs are not carrion consumers.
When offering prey by feeding tongs: a slight wiggle of prey, while moving past the snake, never towards it (that un-nerves shy snakes, as rodents do NOT volunteer
to be dinner in the 'real world'). You want to prey to entice the snake to chase or pounce on it, & most BPs feel safer to do that when they are peeking out of their
hide, & the prey appears to be cluelessly passing by.