Honestly, the best thing you can do is scroll through your analytics and see what videos or at least which weeks you were gaining the most subs and kinda see what you were doing at that time. Maybe, things are getting stale now or it could've been just a sub spike off specific videos. I looked through your channel and don't see anything out of the ordinary, so idk what it could be. Sorry I couldn't be of more help. Hope everything works out alright for you though