Now then, when I got my Bonnie from RSPCA nearly 6 years ago she was recovering from scabs on her back. Had been that bad that RSPCA took her off her owner as she had not taken her to vet and let her get in a right state. Even had to have her tail removed.
I had to keep taking Bonnie back for injections every 2, 3, 4 months for about 2 years until it all cleared up.
She is an indoor cat so I only gave her Frontline now and again to make sure she was ok.
Early this year the scab appeared again, always in the same place. Bonnie had an injection and it didn't help, so I took her back. The vet said Frontline wasn't strong enough for allergies really and told me to use Advocate.
I gave this to Bonnie and the scab went and has not come back since then. Took about 5 days for it to go.
I now give Bonnie Advocate every few months and hopefully it will be ok for her.
Good luck.
Mary