It is the nature of things. Most users just post something the occasional time they find something to post.
Going about finding something to post is the unusual behaviour. I've had users come and go in my communities, and even repeat posters still do so sporadically.
I did notice a lot of your communities don't have icons or banners. That definitely makes a difference when people look for where to share something. No icon makes the community seem dead or like it never got started.
An icon instantly sets a community apart in the results, both in search and the create post page. The sub and active counts are the second thing people look at, after that, I suspect.
I look for things to post on some of the communities I follow. Unfortunately I cannot commit to doing this regularly for all communities I follow.
Also, your icon need not be made by you, so even if your art skills are as mediocre as mine it is fine! Grab some Creative Commons thing, slap it on your community, and attribute it. That is what [email protected] did.