I'm so glad I saw your post. Your daughter sounds exactly like my son did.
Although he was potty trained in the sense that he did not wet his pants by the time he was 2, he never stopped having a lot of soiled underwear. Sometimes extremely soiled. My son's father and I thought this was some kind of psychological problem, because it went on until he was well into his 11th year! I don't need to tell you what a terrible social problem this could cause. I also got tired of scrubbing the underwear, buying new underwear, yelling at the kid, plunging the toilet when he finally did go, - you get the picture. This is problably your life.
We had taken him to the doctors for this and they really didn't provide any useful advice. Fiber, liquids, he needs to mature. We were so frustrated and scared of what his life would be like if he didn't get himself together that I finally went on-line and did some research. I can't remember where I found the advice that worked, because the problem has been solved for years.
Basically, what the "expert" said is that when a young child gets very constipated and has extremely uncomfortable bowel movements, they can become very afraid of the pain and they try not to void again. They do everything in their power to hold it inside. Hence the soiled underwear. Eventually nature (and your efforts to add fiber, liquids, laxatives) will take it's course and they will have to void. That is why it can be days and days between bowel movements and why they are so huge, hard and toilet plugging when they do come! You may also notice your child walking funny - every mom knows what this means...
The article I read said that the simple way to get your child to start going on a regular basis is to utilize glycerin suppositories. Do this on a daily basis until the child becomes regular and ceases to fear having bowel movements. It sounded so simple, but I was afraid my exhusband (who shares custody of our son) would not want to cooperate with this plan. I printed everything off for him to read and hoped he would go along with it for awhile.
It took only one try to get this kid straightened out. I bought a jar of suppositories and explained to him what was going on with him and what was going to change. I told him it should not hurt to have a bowel movement and if he voided regularly as he should, he would experience no discomfort. I gave him a suppository and sent him in the bathroom to insert it himself. (He was way too old for me to do it!) He was pretty uncomfortable trying to hold it in and it acted very quickly. He did not like using the suppository, but it did it's job. I told him that he would be having one of those every day until he began to go regularly and until I began to see unsoiled underwear.
The next day he began using the bathroom immediately when he had the urge to void. I never saw those soiled underwear again. I had fully expected a week or so of suppositories. We were all ecstatic about his success.
I know this seems too simple, but it sure sounds like your daughter is in the same boat my son was in. Of course, he was a few years older than her and we have a very open way of talking about health, our bodies and bodily functions. It was relatively easy for me to try this with him and he had a clear understanding of what he needed to change.
I hope this provides you with an option. Definitely do some research on it on the internet yourself.
It would be great if she got better as fast as my dear boy did!
Best of Luck to you both!