If there's any one that that'll always be unpredictable its human being and their actions and choices. Sometimes the reason for not subscribing could be something really small or simple other times it could be something more but you may never know what unless they say so.
The most common reasons do tend to fall under what you mentioned though:
so that said the best course of action is to keep updating and wait till folks come around
I've actually seen some instances or had it happen myself where I'd take a look at a certain novel or comic, it catch my interest and I'd think I'd subbed to go back and read it later and lo and behold I hadn't done it. Plus if you forget the name of a work finding it again can be tough. So forgetfulness or just missing the sub button happens.
I can imagine for a fact that people who're completely new to Tapas might not know how to navigate the site so they're learning as they browse. It's one of the reasons I'll try to mention things like likes, subs, and comments at the bottom of an episode (even if it's a turn off for some folks)
Still keep at it and even if you might not get those subs right away you can still remember that there's still folks presently engaged and interested in your work, best of luck 