The only way to confirm if there is actually a loss of active subscriber activity is to look at it in a larger perspective, depending on your update schedule and type of comic.
If you're a slice-of-life comic strip that updates 5 times a week, you'll be able to compare week to week and see how many likes, comments, etc. your story is receiving. If you're a plot-based comic that uploads one 40-panel episode each week, you can probably judge week by week and see the ebbs and flows over time. But for a large portion of users here, making 1-2 comic page sized updates, 1-2 times a week, it's going to be harder to see the actual stats, for reasons darthmongoose said and more.
Sadly Tapas doesn't have the analytics tools needed, so you'd have to input the data yourself, but one way to track is to look at a single arc-sized chunk of your story, maybe 3-6 months' worth of interactions, but to view each day on a rolling average and see how that 3-6 month average fluctuates as your story progresses. I'm too lazy to actually fulfill my own stat nerd dreams, but I think anyone willing to do it will often see that their worries are just that they are looking too close at the day-to-day instead of the whole.