Personally I'd respond. The ladies have a lot going on between booking flights, hotel rooms, screening clients and placing ads etc. Then there are all of the things they do in order to get themselves looking smokin; hot. Finally, they also have a "real life"  
I had a lady cancel our first date because she "said" she had the flu and she also said on her twitter she was sick. I gave her a second try and she has turned out to be very dependable so I now see her about every two months. I know another traveling lady who can best be described as "flaky"...she sometimes sends me an email saying she'll be in town next week or the week and then two months later I get a text saying she's at the airport. I take it all in stride because I know sessions with her are always great.
Stuff happens. Yesterday I emailed one of my regulars because she had not responded about our upcumming date. She thought she had, and found the email she meant to send me in her draft folder. And this girl is as dependable as they come