This has happened to me as well. Two-thirds of those who have responded to my ad, even to the point of making an appointment just to meet for coffee and check out the energy first, have disappeared. They don't answer the phone, don't respond to e-mails, it's just life. I've had plenty of experience setting the place up for a shoot, then taking it down again, I've gotten pretty good at it.
And I do love it when someone does eventually call back, I've heard this 'had to pick up a friend at the airport who didn't tell me she was coming' three times. Some kind of friend.
The one I also liked was Jade, a local fetish model, who's stood me up twice, yet keeps responding to my ad. She tried to blame the previous bail outs on me, but when I didn't write back to help escalate the fight, she eventually wrote back and confessed she didn't know if she'd be able to take all that tickling and chickened out, but after having a few experiences, feels she could now. I still didn't respond.
It just be's that ways. But hey, it makes one even more appreciative of the models who do show up.