Lisa, I have a gut feeling that she could be anemic and a slight anemia would not necessarily show up in just a routine blood test. Some studies showed that anemia could also reick havoc on leg (bone) structure; (like a horse with rickets) that is, if she was very well bred and had nice good straight long legs before, that the anemia could off set that bone structure. But also, other defiencies could do that too. This is where Red Cell can come in. Red Cell is really designed for an iron deficiency and especially for anemic horses. Not sure of the dosage, but normally when dosing minis with horse size stuff I just divide the dosage by 4. But do check with your vet on this one, as a touch of anemia is just my best guess here. Also Omelene is also great stuff that I use too and you would just use the Red Cell, which is a liquid as a top dressing.
The other thing too is that she could simply be at this gauky age where she is having a growth spurt and can't keep up with herself.
Last but not least, maybe someone is stealing her hay/grain or she just may be a nervous type girl or is constantly running off the weight as fast as you try to put it on.