It may be that the f/t prey is not warm enough to trigger the snake to want to eat it (remember they normally do not eat dead, cold prey). It may be that you are too close and hanging over the enclosure triggering your snake to see you as a possible predator. When a snake eats, swallowing that whole prey item, it's at a terribly vulnerable time. It can't easily take off or fight back with it's mouth stuffed full of half-swallowed prey. If it perceives it's not safe because you are too close, it may refuse to settle down and eat.

I've had an adult who normally eats very well, back the almost swallowed rat right out of her mouth because I accidentally bumped her tub while she was in the processing of eating. We have another adult female that will strike but refuse to swallow her prey because we are moving around the snake room so she always has to be fed last.