Could she be having an allergic reaction to something?
I use silver lining herbs to great effect. You email Johnny and send him a photograph of both the problem and the whole horse. He dowses the photographs and get backs to you with what he believes the issues could be. All of that part is free. If you agree that he might be able to help he will then make up a herbal mix for your specific horse - that part you pay for!
I know that it all sounds very touchy feely, but my 17.2hh Warmblood suffered terribly with allergies and was always coughing and was covered in urticaria on a regular basis. I had been down every other avenue, vets had blood tested him, scoped him - all sorts, other therapists had done their best too, but the herbs have helped him hugely. I notice if I run out that within a week he starts to cough again and he gets a few bumps. Johnny was also the only person who came back to me and asked - "has your vet ever suggested that this horse could be mildly diabetic" - he hadn't, and yes he is, so once all sugar/molassed feeds were removed from his diet his uritcaria just disappeared!!!!!!!!!!!!!!!!http://www.silverliningherbs.co.uk/
Might be worth an email even if it doesn't take you anywhere - - - -