I'm sure alot of people, like me, are usually busy during the days, and at night they tend to spend time on the server, gaming.
The forums that are "full" typically have loads of younger aged players, or players established long enough that even people with jobs and families still make the time to get on and be part of the community.
The latter is typically a slow process as people need to come to see NF *as* their community, and to gain that sense of commeraderie.
When I was with LKM818 for TF2, we worked at making the FORUM an interesting place. But we did it by doing "LKM BBQ"s where we'd set a date several months in advance, and then all meet up for example at the shooting range on the hill there in Valencia, by the freeway; where they have a BBQ area as well as the shooting range. Oaktree? i think.
People would get to know eachother. And then other people on the server regularly would hear about it, and more of them would want to come to the next event.
We did LAN parties at people's houses, BBQ's, stuff like that. Gets people to know eachother, get interested, and then the FORUM grew as a result.