I know this old as of today, but I managed an account that has 60k of followers in instagram by using the method followforfollow using the iPhone and my account was acting the same way, when someone used to follow me, my followers count used to go down instead of up and I think I found a fix for this, what I've done was to is to put my account to private for 30 mins or so and approve followers from there and after I put it back to public and leave the notifications on for my instagram and it seems to have fixed itself, I have to test to see if it did but I think that is what you have to do from time to time if it does happen again.