Ear Problem

Hi all, I have not been on here for 18mths or so due to moving, selling birds etc.I have 8 birds now, 1 causing a problem. I noticed he was scratching and had a bloody ear, thought he may have caught it on something. A few days later noticed he had a yellow crusty looking substance around his ear, cleaned it off as well as I could, used cotton bud and rubbed in some antibiotic ointment. Thought it had worked but his ear has again this yellow crust. Can anyone suggest what it may be and treatment? He has been treated with Ivomectin about a month ago. Does not have any scale on feet cere etc. ??