My DD has been peeing on the potty off and on since 16 months. This past summer at 2 years 4 months she begged for undies and it was a totall diaster. We let her wear them for 2 months but she rarely pooped on the potty and had at least 1-3 pee accidents most days although had a lot of accident free pee days and started staying dry at naps and wanted to go pee on her own. We ended up putting her into pull-ups in late August and told her that she could earn her undies back when she started pooping on the potty. We decided to us pull-ups since she was wanting to go pee alone and we didn't want to step back with that.
A few weeks ago, she again started asking for undies. She still had not pooped on the potty (had been having constipation issues so we now have her on Miralax daily which totally has helped) but has been doing great with pee, going on her own a ton and keeping her pull-up dry. Last Thursday we took the jump and sent her to school in undies. She had 2 pee and 2 poop accidents that 1st day. On Friday, she finally pooped in the potty at daycare and also had no pee accidents at school and 1 sort of poop accident at home that night. Sat she had a poop accident and yesterday, she had no accidents, pooped on the potty like it was no big deal. We were all so excited.
I am sure we will still have accidents but she is getting it and seems to be getting over the whole poop issue.
What worked for us - not pressuring her, following her lead. We started with Skittle rewards - 1 for pee and 2 for poop. Since she was getting the pee stuff with no issue, we stopped rewarding her for pee and upped the poop reward to 3 Skittles. We also added in a trip to McDonalds (something she always asked for that we rarely ever give in to - we live way to close to one with a playland) for the 1st day she was accident free. We also have told her that once she is accident free or close to it for a few days in a row, we would let her pick out some new undies which she can't wait to do.