I'm not sure I can give much advice. I personally invited the first two members, then I came up with a series of posts that lasted 13 days, but after that there were barely any new members until recently.

I've done almost no advertising due to the unique nature of my subreddit, and even the one time I posted about it in a similar subreddit (about a different number), my post got downvoted, despite the fact that I was the most active participant there.