Some channels I go back to their oldest videos and scroll along to see how they've progressed over time. What I've noticed about your channel is a decrease in viewers in the past 2yrs especially, and I think it is because of how you decided to not have eye catching thumbnails as often as you used to. Having blank thumbnails that don't any colors and nothing to show the finished artwork over time I'd assume is leading to your drop in views. People love to see finished artworks as a thumbnail. It shows the quality of your work, and of course is more eye catching.