Why does my cat have itchy ears with no presence of ear mites.?

My family has 4 cats that remain indoors, around spring time in upstate NY one of the cats started shaking his head and scratching his ears to the point of bleeding. The other cats seemed asymptomatic. We have done everything for this cat, multiple ear treatments, allergy medication, blood work, etc everything has come back negative for ear mites etc. We have gone to cat specialist after cat specialist, dermatologists and everyone is baffled. He was doing well for awhile but was having difficulty putting weight back on but the scratching seemed to stop. Recently it started once again. Does anyone have any advice for us or any experience with this? Thanks.