I am having this problem with my forum forum is 1.5 yr old forum is just starting to members posting now. Here is what my problems are

I have tweaked the forum rules

I had 2 stop forum spam plugins installed I disabled one I think it might have been blocking non spammers

I had a few sections blocked from guest they need to join so they could view these sections I disabled this till I get a good amount more posters

Another thing I am doing is promoting my forum 2.5 times more than before

Got 8,250 black links for my forum

Try some of this things they just might help you get more members to join and post