Did you make any changes around the date that you hit your peak? Did a competitor app come out around that time? The cause of the decline could provide a clue

Did you make any changes around the date that you hit your peak? Did a competitor app come out around that time? The cause of the decline could provide a clue of what (if anything) you're doing wrong. If you have data/contact info of individual users, you can try to email all the users that are no longer using the app asking them why they stopped. In summary- figure out the cause of the decline, then make the necessary changes in response.