I have that problem all the time. The most consistent way I’ve gotten around it is to click on any post within that community. There’ll be a subscribe button in there, too, and that one always shows up for me, unlike the community page’s. I haven’t found a way around this problem if a community has zero posts, so if I don’t have something reminding me to go back, I just forget about it. If you’re finding new ones to subscribe to through New [email protected] (or similar), you can save the post advertising the community and go back to it later.