There are a lot of ingredients to channel success. After a bit of time you notice that some ingredients are more important than others. The biggest thing is Thumbnail quality, TOPIC, and title.
If your views are dropping, it could be that the topics you're choosing aren't interesting to your target audience. I had this issue last year when I went from tutorial videos to speed drawings. My views flatlined because people weren't as interested in watching me invent dinosaurs. I just had to adjust my topics to what worked for me.
Also, an uninteresting topic, next to a thumbnail that isn't very interesting will mean people are less likely to click. So when your views and subs slow down, it's time to start analysing your videos, analyse your audience retention to see if people are jumping out of your video very early on indicating maybe your topic isn't interesting compared to your topic and thumbnail. Time to start analysing your thumbnail, because if you have low CTR, regardless of your impressions, it means people aren't clicking, and thumbnaills are super important in getting a click. They say you shouldn't judge a book by it's cover, but we all know if a newspaper doesn't have an interesting cover story, people won't go and look at it, and some with thumbnails. If the thumbnail looks uninteresting, people aren't going to click on it.