Bulldogs World

Looking for some help. This is my bully, Nelly. She's been starting to lose her hair and it's only gotten worse and is currently on both sides. I have had her thyroid tested and tested for mites which both came back negative. It's not a rash or infected and she isn't bothered by it or scratches or is itchy. I switched her to grain free food earlier this year but hasn't done the trick. A dog dermatologist also suggested for her to take melatonin (with out being seen) and that hasn't worked either. I am hoping one or some of you have had this issue and was wondering what helped your bully as it's breaking my heart she is losing her beautiful coat :( thank you in advance! Pictures attached.