Mites Still scratching after revolution?

Hi so about a month ago I took my guinea pigs to the vet because they were scratching quite a lot, and the vet tested for mites bu testing a few hairs from each guinea pig but nothing showed. She gave me revolution to treat them with anyway. She treated them there and she gave me some extra to do the treatment again on my own two weeks later. She said they may not stop scratching until about a week after the second dose but now it's been over a week and they're still scratching? Is it possible that they're just itchy really often? They've been losing hairs but it's just like normal shedding, it's not in patches and there are no bald spots except in the centre of where their rosettes are (which is normal, they're not really bald spots, it's just where the hair starts to grow, I'll see if I can attach pics anyway. But is it possible that they're just itchy quite often? They don't really have any other symptoms besides itching, they're still eating and drinking fine and their weights are fine.