As one who posts on occasion, my opinion as to what would help encourage those who don't post often::

When someone posts who doesn't post often, at least ACKNOWLEDGE that they've been heard by someone, and give them some type of comment!!

Don't just allow their post to float out there like no one noticed or even cared about whether they took the time to make their comment or not-- as the usual posters go about their discussions. This wouldn't usually take more than a sentence of effort.