I set up my moneymaking forum recently.I submit it to many social bookmarks and social dirctory.I got about ten visits everyday.But they rarely register at my sites.What’s the problem?
You should assured your forums that it is informative and will be useful. Like having a good and update topics about your niche. 
I am assuming you setup this forum without having a website to back it up (and therefore are working with no initial members or contributors), you have put yourself in a very bad position as there are literally millions of forums out there and the only ones which get traffic are the most popular. Without an active user base from the offset your forum will remain dead with occasional visitors, it’s one reason why very few people manage to turn a forum into something as successful as this one. 
Forums can be tough. If it’s a rather inactive forum people don’t want to sign up because they may feel there will be lack of responses. You could hire paid forum posters. They do a great job and for cheap, based on my experience.