I remember we had a similar problem with overall event attendance and I think we discussed having those members with low attendance plan an (appropriate) event for the chapter to attend together. That way they would (ideally) understand all the work it took to plan an event, get them involved in the chapter more, and if not many people showed up they would feel how the executive officers feel when there is low attendance. We didn't actually try it though so I couldn't tell you how it turned out. :P
We also rewarded those who were above 95% at the end of the semester (although only 80% or something was required). That way you're focusing on the positive with incentives instead of the negative with consequences (carrot/stick concept)
