Gaining followers on Instagram means your photos and videos are seen by more and more people every day. As the numbers grow, you may have a harder time keeping track of them all, or you may miss some you were sure were there previously. Some followers may be missing due to limits in Instagram's programming or through voluntary withdrawal from your feed.
Watching your follower list is easy when it's in the low double digits, as it's quick to see number changes. Larger lists, however, don't allow you to check them entirely, as Intragram limits the list you see to the most recent 200 followers. So, although you may have 1000 followers, you'll only see the 200 who've followed you last.
Un-followed, Private or Deleted Accounts
Even if you keep meticulous records of everyone who's ever followed you, the number may change due to the personal preference of the accounts' owners. Someone may have decided to un-follow you, or change his or her account to private and essentially become hidden to those not allowed access. Others may have deleted or disabled their accounts entirely, disappearing from your follower list.
Under Instagram's Help Center, buried in the “There's a Problem With” section, the company admits there is a known bug that affects the accuracy of your follower/following list count. This bug could appear when some of your followers have recently disabled their accounts, but the site hasn't updated the information and removed them from your follower list. Instagram knows of the bug, but has no official projected time for resolving the issue.
Keep Tabs on All Followers
Although Instagram doesn't offer you a way to keep a list of every single follower you have, the site does recommend a few third-party sites that can do this. Instagram suggests signing up with a site such as Webstagram or Gramfeed (links in Resources) to keep better track on everyone you follow or who follows you.