Make yourself a normal account on your forum, just like all your signups would have, and make sure that it's working. There may be a problem in your codes that prevent them from posting, you need to go through manually and see what they see if you haven't already. Then make some conversation stimulating topics yourself, or hire a paid forum posting company to get a few threads started. If your forum looks populated, even if it isn't yet, then your signups are more likely to start talking.