They flake because it isn't a "strong number".
The whole point of the number is so that you can meet again and carry on where you left off. If you can carry on spending time when you first meet that is the ideal.

If you can't carry on spending time together, i.e. move to another bar/club/eatery/walk/party/your place .etc, then you get the number.
So what is happening is that in the moment she felt it was on and was comfortable to give you her number. Later however she thinks back and if she doesn't feel a strong "connection" she flakes.
Also bear in mind the influence of alcohol and how strong her memory of you actually is. A 5 minute interaction where she gave you her number isn't going to be a strong connection where she is compelled to see you again.
She may have given her phone number to many different guys..
The quality of your interaction is determining whether she flakes after giving you here number.
I've had many not respond, so I speak from experience