You say "we" so I presume there are multiple people behind the site?
In which case, how much effort are you putting into posting on the forums yourselves?
Simply that firing up momentum on a forum takes time, and you need to take the lead - by starting conversations yourselves, and opening up areas of discussion you not simply want to encourage, but also want to make look inviting for others.
Anyway, see if this article from the Platinax Blog helps:
http://www.platinax.co.uk/blog/44-successful-forum/