Pages/Site Not Ranking on Google Results As Expected

About 8 months ago, we set up RankMath Pro on one of our sites, and generally kept an eye on how we ranked on Google search results. Obviously it took us a bit to move up in the rankings, but we eventually ended up on the first page for the specific keywords we wanted to rank for. However, this past month I noticed that we’re not even on the first three or four pages on Google search results. Any ideas on what could have gone wrong? Perhaps I did something wrong? Any tips, suggestions, critique, etc is welcomed. TIA!