I think spotty participation is pretty common as people have different interests and commitments. One year for special projects we did a ostrich round robin. Called ostrich because it's a bird that doesn't fly, so you don't pass it around. Basically you start with a block or UFO and work on your own project adding a round each month. I would try to accommodate different skill levels so each month there were two options like add a border with new colors or a new technique. Then give two options at the next month for 5 to 6 rounds. One of my friends made a very beautiful quilt and she credits it's success to a round where one of the choices was use a different shade (lighter or darker) of a color you have been using.