Hi L.,
Two small bits of advice: Try to monitor the timing. Is this happening at about the same time every day? (Like after lunch, or after breakfast, something like that.) If so, then I would take her to the bathroom every 20 minutes for about two hours surrounding that time. You will need to re-train her. Also, although this is hard, and I'm going through a lot of this with my 4.5 yr. old daughter who seems to be having (at least) pp, and sometimes, poopoo accidents on a daily basis (poopoo maybe once/week), when she's been trained for about a year now, do your best to keep as positive as you can. Make a big, sincere, deal about her doing ANYTHING in the potty on her own. As best you can, try not to show disappointment in her when she has an accident. Make light of it, let her know it's okay and you'll just clean it up, get a fresh pair of underwear, etc. Let her know, daily, that you know she's going to be able to poop in the potty very soon as she's growing and learning so much. Whatever you do, just stay positive. Although, for the most part, she probably doesn't have as much control as you think she does over this, she is getting attention for it (even if it's negative). Turning your attention to all positive things she is doing will help. Also, sometimes this can be a power piece. There is so little that kids have power over in their lives that, sometimes, they will find anything they can control, and do so. However, even with this, the answer is not to assume they can control it, but, rather, give them power when you can in other situations. Even empower her with the potty.
Okay, that wasn't so short, but, hopefully, it will help. If you have any further questions, let me know. I know of some good resource websites, too. =0) Sometimes they've provided helpful info. Mostly, just know that it may take some time. Oh, also, have their been any changes in your lives that may affect this in any way? You'd be surprised how children react to things, even inadvertently. If there have been job changes, added stressors, etc., any of these can, and usually do, have an affect. This is the case in our family (husband lost his job and, although I run a preschool, it's a VERY full time job, and I also run a business, so....).
Good luck! =0)
L.