DeepBlue: I noticed that I myself have occasionally "flaked" with women (flaked from her point of view) by not calling her after I went to the trouble of asking her for her number. I've talked to women about this, and they definitely DO feel rejected when they have given a guy their number and he never bothers to call.
By looking at why I was "flaking" with them, I gained some insight into the reasons why women flake as well.
So, the basic scenario is that I'd go to the effort of getting a woman's number, and I'd really be into her, convinced that I want to get together with her, but a day or two later I'd lose interest in calling her (or in emailing her if it was an @close).
The problem in a nutshell is that when I'm getting her number then there are two major factors "supporting" my desire for her:
The "so close to sex" factor. The fact that she is right there with me in the flesh makes me feel tantalizingly close to the possibility of having sex with her--because there is the very real possibility that we could go off somewhere right then to fuck. It's the difference between having a delicious dish in front of you where you can smell it, versus someone telling you over the phone about it.
There may be a number of "extras" involved in my being turned on to her. Seeing her body, talking to her, maybe it's dark, maybe we had a drink, etc--all these things increase my desire for her at that moment.
The next day, both these things change. That exciting feeling of imminent, immediate sex is gone, and all the extras are absent as well.
So suddenly I'm asking myself whether I'm "interested enough" in her that I should bother pursuing her by calling her and setting up a get together.
The point is, this question of whether I am "interested enough" never came up when I was with her. Why? Because in that situation my interest was naturally present, supported by all the extras, by her presence, by the feeling that sex is potentially imminent--and so I never even feel a need to ask myself whether I'm "interested enough"--I just flow with the desire that is naturally there and I get her number because of it.
I'm convinced that it's the same for women. They give out their numbers because they genuinely are interested--right at that moment, but their interest is heavily supported by surrounding factors--by all the extras, including you being with her physically, face-to-face.
When the "supporting factors" for her desire are suddenly absent, and you call her, that triggers her asking herself a question that becomes an obstacle--namely asking herself whether she is "interested enough."
One solution for chicks flaking is to do what Rock Harders has suggested, which is to take things as far as possible with her right then, in the heat of the moment, by going somewhere else with her, and somewhere else again, till you end up someplace where you are kissing and having sex.
Another solution is to get her number, but arrange the next get together in such a way that it saves her from having to ask herself "am I interested enough in him".
You can do this by temporarily dropping back to having some "other reason" for getting together (something that doesn't revolve around you being interested in each other) and you can count on the fact that once you are together, your presence, face-to-face, will reawaken her interest so the "obstacle question" about her interest level never comes up.
Another approach is to not only get her number, but also find out in advance if she'll be at some club or pub or wherever and then manage to run into her there. That way, you'll not only have your live presence working for you, but also the "extras" provided by the setting.