I feel like there are multiple issues here. Am I correct when I am guessing you are the user Sourcandy on the tapastic website?
If you are, then your biggest issue is that you have over 100 subscribers split between 17 comic series. It takes some time for readers to get invested in your stories. They have to get to know your characters and the story you are telling. For that to happen, a series has to be updated regularly - or a lot of content has to be posted.
Would you comment on a series with 3 pages to read? Probably not. Instead you might subscribe, continue reading and one day you might feel like leaving a comment to a particular scene. That is at least how it works for me and it seems to be similar for many of my subscribers.
I would concentrate on one or two of your most favorite comics and build a community around them. Eventually you will see more responses from your subscribers.