OP, could be one of the reasons below:
1. Instagram's algorithm limiting new followers
2. Instagram catching more bots, so less new followers
3. MP licenses coming to an end, so less new followers
4. Your posts not as HQ, different hashtags from before, or not hitting top of hashtags used.
I did notice less followers for the accounts that I handle from clients, but that was in June. Now back to normal and going strong every day. Just keep at it, quality over quantity.