My 2 1/2 year daughter was already toilet trained at 2. She knows how to tell me that she wants to pee & she used to be able to poo in the toilet. However recently she only poo on her diapers. She will wait the whole day & not poo until I put her on diapers for naptime. Then she does her thing & disturbs her nap.

My daughter is exactly the same except she's nearly three. I just give her a nappy anytime she needs a poo throughout the day. I figure she's got till she's 18 to get it right. How many 18 years old wear a nappy when they poo?! I'm not worried about my daughter and neither should you be. Relax!
She'll probably get it when she goes to school/nursery. You may cause more problems by trying to force her to poo in the loo before she's ready. (constipation?)
Try backing off a bit and let her poo where she feels comfortable for a few months, then suggest the loo again, but don't force it.