It could be an overlooked medical issue? Has she been tested for UTI's and such? Also, my cousin used to wet the bed frequently and it turned out she had some kind of issue with her urethra (narrow or something like that) that was quickly resolved with surgery. Remember, the squeaky wheel gets the oil, and if you make a big stink at the doctor they will do more tests and find out what is REALLY wrong. Just keep telling them, I know something is wrong and I want to know what it is. She's 13, and she should have outgrown it by now.
I can't say that its definitely medical though, as I'm not in your shoes and I don't know your daughter. I do have to share one other experience as well, my younger brother wet the bed until he was 12 or 13 and they thought it was some problem or whatever, he was on medicine too and everything else. Finally my mom decided he needed to stop and every time he would wet the bed she would make him get up, take all his bedding off himself and wash it all himself and scrub his mattress. He hated it. And you know what? He stopped wetting the bed. Didn't take long either! He also had/has a lot of other serious behavioral issues though and I imagine that was part of all of it.
It can be hard to tell with kids, what is really the problem. But rule out anything medical first! Good luck, I hope the issue gets resolved so the poor thing can have sleepovers.