I noticed your series follows a narrative β so my best guess is that up-to-date readers are letting episodes pile up for binging later, and new subscribers just haven't caught up yet. You have a lot of episodes out, so that will take a while for new subs to read them all. Especially for those who are subbed to hundreds or even thousands of series. People aren't going to view your weekly updates if they are still stuck at the beginning of your series. Just keep that in mind.
I wouldn't stress about this though. The fact that you are gaining subscribers is a good thing. Your art looks nice and appears consistent, so I don't think that's the issue. Maybe your story took a turn that your readers don't find interesting anymore? Having never read your comic though, it's hard to say. The only way you could really address this is to put a post up on your page asking your readers for constructive criticism. That would give a bit more insight as to whether or not anything needs improvement.