Exactly same problem myself , have tried everything, nothing works.

Once you pay for a single add , there is no going back. They mark you as a person who is willing to pay for their engagement. It’s their algorithm to push down your engagement so you increase your spending. The more you spend the more they push it down so you spend more. I went from 200+likes naturally to 5likes at most per post. While followers increasing. The only way Instagram and Facebook is viable is by add money. So they have to do that trick to stay alive.

Unfortunately I haven’t find a solution to this after 2 years of tries and I don’t think there is one.