when school started and my girls moved in, i had them fill out this survey to see what kind of this things they liked..
you can try it with your residents if you want, I'm attaching it..
food also helps, but its kind of like a bribe and usually they just come, take the food and run.
do your resident's know each other really well? it could be that they've become comfortable in their groups of friends and don't want to venture outside of what they know.
also, if you have to have a mandatory meeting for some reason.. or you just make up a reason to get them all together.. emphasize that it's mandatory so they get there and then just have some really fun activities planned. we've done that here and it's actually worked out really well..
hope that helps, let me know if you need anymore help.