G.T.
do you give her a bath before bed? A lot of kids will poop then since they get relaxed. she is getting relaxed in her bed and is doing it there instead.
Maybe run the bath, ask her if she needs to poop, if not put her in the tub. After a few minutes in the tub ask her again if she needs to poop. (asking her will make her focus and she might discover she does need to go). If she still doesnt go after her bath.... WELL... I dont know what to tell ya. Too bad you cant get her to go in the AM instead of the PM, but everyones bowels are different. She sounds like she likes to be alone when she poops and thats what she waits for.
Maybe putting her on the potty in the evening with a book and leave her alone to poop might work, sounds like she doesnt like distractions while doing the deed.