When you're posting on the forums, what else are you doing, or trying to do? Are you working at the same time? Do you IM/vent? Do you watch TV? Are you cooking dinner? Do you focus on posting and nothing else?
For me, I'm easily distracted (ESFP much?) so I always am doing something else at the same time but I find that I'm terrible at this, and my foruming suffers. I keep doing it though. I'm finding that I need to take time out from everything else just to focus on reading the forums if I want to really get it.