S.W.
You are not alone! I am having the same situation with my 4 yr old daughter. She is doing wonderful with peeing in the potty but holds the poop until I put her in a pullup at naptime or bedtime. She will even TELL me "I'm pooping" but refuses to do it on the toilet. I am very proud of her progress (this has been a LONG process) but do not understand why she does this. Power struggle? I am anxious to see what advice you get!
Update: I tried the "poop party" idea that another poster suggested. Well, I am astounded because it worked!!! My DD pooped in the potty! We will see if it continues, but at least she has had one sucessful poop. a BIG thank you to whoever suggested that!