Many people book performers hoping for them to have a decent draw. For instance, if you book five comedians, you generally expect each to bring around five fans so you can have a nice show with 20 to 30 audience members.
But at some point, a performer's doing a show each night (or even multiple shows), so are they still expected to draw a crowd to every one of their performances? How do you handle this, as both someone who books variety shows and also as someone who performs at many shows?
Are you a club bringing in a big name guy? Then sure, you can expect him to have a decent draw (presumably in proportion to how much ya pay him). But for the kinda shows I do, I think it's the job of the producer to get people in the room. I don't think you should expect comedians you book to draw people (unless you explain that when you ask them to do the show).
Now you can encourage comics to bring friends and ask them to promote the show online or at Facebook. But I don't think you should be upset if they fail to bring anyone out. And that's especially true if that person runs their own show (it's tough enough to promote your own show, much less other people's shows you're on too).
If you can't fill your room consistently, get a smaller room. And if you can't do that, maybe you're better off hosting a mic where you'll (hopefully) get other comics to show up at least.