We had exactly this problem with our blue roan,Poppy, when she was 2. We kept going back to the vets and had different drops and then she had a general when they carried out a 'debride' and had to wear a contact lens. However this still didn't solve the problem and we were referred to a specialist at The Willows, where they have super duper equipment. The eye specialist there quickly diagnosed an in growing eye lash inside her eye lid, they removed it and scraped the inner eyelids (yuk) to prevent future reoccurrence. Fingers crossed she has had no problems since, touch wood, that was about 3 years ago.
Apparently this problem is more common in blue roans. If I was in your shoes I would ask for a referral and get a more specialist opinion. ❤️