One fix to a problem like this is of course to recruit more people, get your roster to about 45 and you should never run into an attendance problem. With no explanation as to why your attendance is slipping I'm not really sure how to answer you. Is your raid leader an asshole? Is there no motivation to raid? Do you offer free repairs on progression bosses? Have people not been showing up because of school just starting? If I know more I may be able to make more suggestions. Open a dialogue with those that didn't show up and see what the problem is.