Huh...Same amount of readers since last year you say?
That's a new one.
I don't know if there's a "threshold" per say, but I will say that its probably not that you aren't trying hard so much as it is your comic simply slips under the radar of the majority audience that visits Tapastic. Most people have the attention span of goldfish especially so in the era of social media and smartphones and as such, they will read a few comics here and there and then call it a day. That's simply how it is and why for example slice of life 4 panel comics are so popular here. (Not that they are necessarily good mind you, but its like fast food for the brain so to speak. As the slogan for McDonald's say, 1 billion served!)
Another thing it could be is that people overlook your comic because they feel its too similar to other comics they read. Time is valuable to a lot of people and if they don't feel your comic offers anything they can't get from a similar comic they are already subbed to, then so be it. That's just a fact of life as a content creator. It's why I've cleaned out my reader list every now and then. Too many comics, too little time.
Lastly, as somebody who makes a niche webcomic about Asperger's Syndrome, I just want to pitch my two personal cents in and say that there's always going to be something you overlook or simply aren't aware of as a web cartoonist. Maybe your anatomy really is that bad? Maybe everybody speaks in snark even if it wasn't your intention? Maybe you aren't promoting yourself or engaging with fellow creators enough? Whatever it is, I know the feeling that you feel like you are doing everything you can to let people know your comic exists, but sometimes, you've just got to keep chugging on ahead and hope for better when it seems like the entire deck is stacked against you.
I've gained people who I never though would be interested in a little comic about autism. More than I thought I would starting out, but also, not quite the number I was hoping for either on some days when I see other comics doing exponentially better than mine despite not seemingly having anything to write home about. In other words, its a mixed bag to me at times, but again, you've got to keep chugging on ahead no matter what and see where it takes you in the long term.