Has she lost any weight? Aubrey, our bully before Aubie, had problems with eating and throwing up off and on, and it turned out she had an uncommon intestinal problem that went undiagnosed. It didn't show up until the problem itself caused a blockage that showed on an ultrasound. All I can say is try whatever you can to figure out the problem and don't take no for an answer. It could be many things and you just have to narrow it down like you are. If the food works, hey, that's great! Maybe then you can branch out to a somewhat better brand that's for sensitive stomachs.
I wish I could be of more help, but I don't think my experience quite fits yours with Ellie... at least, I really really hope not. It'd be great if our babies could tell us what was wrong, huh?