There are a number of reasons some of your followers may no longer show up on Instagram. You may have lost some followers, there may be an issue that Instagram knows about or you may have hit the limit for how many followers Instagram displays.
If you aren't seeing some of your followers, but the number of followers you see matches your follower count, it is possible that some of your followers have un-followed you. Just because someone started following you doesn't mean he will continue to follow you.
Follow Count Bug
If there is a discrepancy between the followers you can see and your follower count, the cause could be a known Instagram error. If some of your followers' accounts were recently disabled then Instagram may not update your follower count accurately.
If you have more than 200 followers, then Instagram will not display every profile following you. It will only display the 200 users who most recently began following you.