I personally like to keep a number of projects going at the same time now. While I was doing Incubus Tales, which I did for 7+ years, I did a few side projects, but none of them really took off. I usually give any project at least a few pages to see if I can "feel" them even when I'm not particularly inspired or in the mood, since logically if a series continues over a period of months or years, you aren't going to feel super into them every single day. Some of the stories I did also had a theme that was interesting at one time, but became uninteresting to me looking back later, or ended up being one of countless others because a trend hit, so that can also have an effect on how a series feels.
I feel like if a series isn't performing the way you want it to, take a step back and see what you can see -- try to look for what isn't working for you. Sometimes it helps to take a dry erase board or a text editor or something and make a chart or bullet points, whatever works for you! See if you can list what is working and what isn't, and look at the two things, comparing and contrasting.
As others here have noted, it can also really help to ask your readers and subscribers what their thoughts are on it, but one thing I'd like to emphasize is to ask people what they like rather than what they don't. It's much easier for people to think of things they dislike, and if you end up getting rid of everything someone dislikes, you'll have nothing left that's yours. It takes more effort and forces people to think, if they have to think of what they do like and make a list of it.
Ultimately, it all comes down to you. This is your call. If you're having a really hard time coming up with stories, if you don't feel anything for the story, if you have no connection to the characters anymore, it may be time to rest it. You don't have to end it or put it down forever, but maybe it would be a good idea to come back to it at a later time. I do think it's best to be honest in these cases though, and if you aren't sure if you can come back to it, just say so. Most of the people who grew to love your style and approach will come with you to the next project. Just be sure to keep them posted about what that is!