Does she complain that it hurts to swallow? Or ask if she can feel her food going down her throat and into her stomach. If it hurts to swallow or she can tell you exactly where her food is in her system, it could be a very real disease. Do a google search for achalasia. It is a rare disease, but can effect any age. Many times it goes years untreated, especially in children. I was diagonised five years ago and my mothers best friend about 10 years ago. So, though it is rare, they are finding it in more and more cases.
My Drs did a scope, and upper GI, and rulled out almost everything. He then said he wanted to do one more test just to rule out this strange sounding disease. I didnt think anyting about it, I figured I was going to get a no, we still dont know what the problem is but to everyones surpise I was one of the 1 in 200,000 with achalasia. Just knowing I was not crazy and there was something wrong was a huge relief. The best part was going back to my family dr and telling him I was not crazy. He was shocked when I told him I had achalasia. He said it is one of those diseases they spend an hour on in med school knowing they will probally never see a real case.
It is hard to describe, but basically it is a cross between MS of the throat and the reverse of GERD. It is not life threating, but it is chronic and will never get better. The nerves in the esophogus are dying and as a result misfiring. It causes food to get "stuck" in your esophagus sometimes going up instead of down. It can be very painful and when I am having a bad day, I know where every bite and drink I take is located all the way until it makes it to my stomach. Very strange feeling and for someone young, it could cause a phobia.
Even if they can rule out "everything" including achalasia, if she is still having trouble, keep fighting a mom knows when something is wrong and your instict is better than any medical test.