I've attempted to schedule shoots with cosplayers that I like their work. Those rarely work out...
On the other hand, when cosplayers contact me and show initiative, they also are generally more motivated to make the shoot happen at the convention.
Maybe the next time you make a forum posting about offering shoots, add some ground rules. You might not fill up your list and will probably scare away the lazy and inconsiderate, but it will also help you find the ones that care enough to show up with their A-game (Wouldn't want to work with people who don't anyways).