As it sounds like the girls are attracted to you I would say its one of two things:
1) Not enough comfort. As you said you can try and talk to them for longer or try and take them on an instant date etc.
2) Maybe your 'vibe' was off. So when she looks back onto the interaction she feels as though there was something not quite right about it. This could be down to you being nervous or not 100% comfortable in the interaction which could result in you being stuck in your head and not being present with her. To be honest though, this one mainly comes with time as you get more experience with all of this you naturally get more comfortable within the situation as it becomes a natural part of you. This one really is so important as if you are completely comfortable she will be as well meaning she can relax and be herself around you.
We can always improve with this but also remember that a lot of the time the girl will flake due to something completely out of our control. E.g. she has a boyfriend, her friends talk her out of it, she is busy, she gets nervous etc (especially with the younger girls).
Hope this helps