You say she shows no signs of stress but that's exactly what not eating is. Sounds like her temperatures are too low, & also you should refrain from handling
any & all new snakes (any age) until they have fed easily for you 3 times at normal intervals. "15 minutes" may not sound like a lot to you, but to her it is...
her whole world changed inexplicably when you took her home...she cannot talk & tell you how terrifying & confusing that is. Assist feeding is making that
worse! She may never have fed for the shop owner, but if she is healthy she should feed for you, IF you provide the right environment, including enough heat
(but thermostatically-controlled to proper range for BPs) and secure "hides" (of the right size for her) and refrain from handling, especially from "assist
feeding".
NO handling! it disrupts her instincts to hunt & eat...the only thing that picks up a snake in the wild is a predator about to eat them...right now, that's what
your handling means to her...leave her alone, let her be a snake first...she needs to feel secure & have privacy & the right temps. (& humidity).