I discovered a while back that Reddit also has a hidden metric beyond karma that affect post visibility, called contributor quality score (CQS). If your CQS is too low, you're deemed a spammer and either can't post in certain subreddits or your posts are automatically hidden.

I'd never heard of this and looked up my own account's CQS. Somehow it was tagged Low.

I'd been a member for years, posted mostly comments with a couple of posts – some of which did quite well in terms of upvotes. I can only imagine that my intermittent posting or the submissions that never got any attention were to blame. Either way, I decided it wasn't worth the effort.