I only wanted to add the I am not sure about your dog but you say treated for mange.... my dog has mange and he has been treated for 6 months and still will be until it is gone. No way of guessing when that might be.
Skin scraping do not always find the mites, Blaze had 8 scraping and found 1 mite. But he has it forsure and I tell you it is a bad thing to watch and not really be able to help.
Benadryl did help for Blaze and he has 2 per day from the time he was 9 weeks old. We hope he grows out of it on his own but sometimes they don't. Good luck and hope you find out what is wrongbut again mange takes a long time to heal and they hack every 21 days and even the died mites drive the dogs nuts.