As others have noted, I think the label inactivity is a big source of the problem. There isn't much happening at Hypnos per se, and, as a result, there is nothing driving new people to the forums.
But another issue that is easy to overlook is that there are numerous places for people to chat and learn about ambient music these days. The forums were a good mechanism for internet communities in the "old" days, but I think Facebook and similar outlets changed that. Ambient is alive and well, but people have multiple channels to communicate, learn, and exchange ideas. For example, I get most of my listening leads from Bandcamp these days--simply following people with similar interests and checking out the things they are buying. And I assume artists have less incentive to interact with the community on forums when they can update their Facebook pages with studio pics, new releases, and have immediate interactions with people who are seeking out their music in the first place.
I like the group here. But I do wish it was more active. People seem to fade away as they get busy with their lives. And, perhaps because it is an easy forum for exchanging ideas, people also tend to piss each other off and become reluctant to come back. Some people seem quick to offend, and others seem quick to take offense.