Hmm well, there might be a couple of reasons. Particularly during spring and summer people are getting out and about so, are you noticing this trend spiking during the spring and summer months? (Because if it is during those time periods, part of the reason may very well be vacation seasons and such.) I'm not saying that's 100% the cause, it's just that during this time on average readership falls because people are out and about.
I read through some of your comics from beginning to end and the one thing I did notice was that the art hasn't changed too drastically. This isn't a bad thing, it's consistency. You might want to work on some elements of cartoon anatomy and shading here and there, but honestly your style and coloring format suit each other well. Sometimes you just hit peaks where you win some, you lose some. You don't have to give up on your dream of being a professional. Maybe just try out some new art techniques to make your characters pop a little more in their environments? Don't worry, I won't leave you high and dry for this bit of advice.
Here, these are some tips for bringing out the shine: Stylization. Now, you've already got your characters established but, I've actually found the advice in this article to be very useful in many aspects. It might help you round out a character, or just give you general ideas on how to apply your skill roster to another part of your work, like backgrounds and such. Either way I find this helpful to have on hand. Backgrounds: These are just some general tips for backgrounds on the whole. I find it to be helpful because it breaks everything down into simple shapes and builds them up. This way, if you ever want to add more to your backgrounds, you've got these tips on hand. 
These aren't a requirement or something like that lol. Your art is already pleasing to the eye, but maybe adding a little bit of pop here and there can help with the trouble you're having? I hope you can use these links and that they're helpful. You're already doing pretty good in my book so, I'd say just keep making the work that makes you happy. Art is a hard thing to make a living at but, it isn't impossible. Heck, you might even try designing some T-shirts and setting up a shop (if you haven't already). Maybe ask your fans and see what they're interested in. 
Best of luck!
