My cat has been known to have the odd bald patch... Sometimes when she's had a scratch or nick when the scab comes off there's a small bare patch left for a while: I guess this is what your vet thought your cat's problem is.
She's also been known to overgroom: she literally licks the fur off herself. At one time she was naked on belly, base of tail, elbows and armpits. Rather embarrasing to take to cattery...
The problem first became really noticeable when my other cat became seriously ill. The vet diagnosed eczema (though the skin mostly seemed fine with just the odd iritation) gave me cream for her but it didn't do any good: just drove her nuts and made her groom more if I put it on! So I only used it if the skin became really iritated, and the bare patches persisted for a couple of years. The turning point was when she had an allergic reaction to some food and ended up with scabs all over her face. Not nice, but the vet gave her a steroid injection which seemed to give the hair elsewhere time to grow back as well. She's now fully covered again, aside from the odd scab and resulting small bare patch...
So... if it persists maybe you should try and work out if there's something that could be stressing your cat out? And see if your cat is grooming obsesively: mine seemed to almost go into a trance sometimes and was hard to distract. If your cat is an overgroomer I've heard a change of scene, eg a trip to cattery, can get them out of the habit. But it didn't work for mine... Hard to say for sure, but I do think it was the steriod injection that turned the corner for her.
