Gaining subscribers is hardly an exact science, but here are some questions you can ask yourself:
1. Have you updated frequently in the last month? I notice a dip in views/subs during those months when I go on chapter break, because every update is an opportunity to reach new people.
2. Is what is happening in your comic right now, in terms of plot, going to grab people? Are you in the middle of a cool action-scene, or exciting intrigue - or are you just moving your characters from point A to point B? Some things are going to be more attention-grabbing than others.
3. Are you promoting yourself via social media other than the Tapastic-forums? Twitter's a popular hangout spot for webcomic-creators.
4. Finally, are you improving the quality of your comic? No matter how hard or how well you promote a comic, if potential readers go to check it out and are disappointed or underwhelmed by it, they won't stick around. No one is amazing from the start - I know I have more than a few terribly drawn and badly told comics hiding in the dusty recesses of my computer - so all we can do is strive to be better. I had a quick look at your comic, and I can see that your lettering is a bit hard to read sometimes, and it doesn't look like you plan out where the text is going to go before you draw the pictures. For example, there's a panel in your latest episode (the one where there's someone saying "No, I was a ninja too", etc. - and you've had to write the text vertically, so readers have to tilt their heads to read it, because it looks like you didn't think about how much space the text would take up before you drew the panel. You're also missing punctuation here and there - missing question-marks and commas, etc. That can be fixed if you get someone to proof-read your comics, though!
5. It's okay to not gain subscribers every time you update. You keep on doing your comic, and you keep on trying to do better, and value the subscribers you already have! Webcomics is and always will be something you're successful at if you're patient.