I wonder whether the problem is compounded by the fact that artists typically start work later than writers, by necessity, and by that stage the writers have their support networks established, making it that much harder for artists to join in?

I agree, it's not the comm itself, and it's not a mod/organiser problem, it's up to us participants- perhaps some of the artists like yourself could organise some friending posts or chats around the time of the matches? That would be a good reminder for us writers, too, that there's a whole vital section of the community that is in danger of being ignored!