Our Guild BOM program is designed to garner blocks to use in our "emergency quilts"--which go to families that lose their homes to floor, fire or tornado. We ask that members make one block for the emergency quilt and one for the member raffle--each month someone that has donated blocks gets the member raffle blocks. We did see a decline in members doing blocks but that turned around when we 1) went to simpler blocks that are quick to do 2) went to all scrappy (although our chair does suggest "spring" colors, etc) Many members will put all their blocks in the emergency pile and only one in the other. Or if they win them, they hand them to the emergency chair to use in quilts.
What didn't work--was when the chair picked blocks that were all easy, but she'd alter the directions to include embellishments, etc and people got frustrated and quick doing them.
What also worked in past, the chair would have pieces of fabric to use for a quarter--so for instance a block may have a focal fabric (25 cents) and then work from stash.