Time to start with one food at a time. If potato works and goes down (and comes out) okay, feed that for a couple days, before adding salmon. Then feed those together for a couple days, and if she is okay, you can try the "salmon and potato" food which likely has other ingredients. It is slow, and not very exciting, but the only real way to figure out what her allergy is, if she has some.

I have been there myself.