Flakey behavior is often the result of an individual who has an abundance of options with little investment made in obtaining any of them. This is just as true for men as it would be for women.
Imagine a woman who has an unbelievable hankering for a piece of chocolate. In front of her is an entire bowl of M&Ms. As she grabs a few M&Ms, one drops to the floor. That woman will make no effort to invest in the energy to bend over to get that M&M because there are plenty more in the bowl right in front of her. But, if you take away that abundance, and there were only 1 M&M left in the bowl, that women just might put forth the effort to chase after that dropped M&M because there are no others so easily within reach to satisfy the craving.
Now, obviously we cannot make other men disappear to eliminate abundance in so that a woman will be enticed to put more effort into pursuing that single treat (you). However, when it comes to pick up, cold approach situations, etc., there is one element that is so often lacking in these situations. Call it the "there is something different about this guy" factor.
Imagine now that a woman has a craving for a big, fat, peanut M&M, but there is only one peanut M&M in that bowl full of regular little M&Ms. She reaches for it and it accidentally drops into her lap in between her legs. That woman is going to take a little more effort to shift herself a little to reach for and find that peanut M&M because she knows it's the only M&M from that bowl with a peanut in it.
Technically, you may be doing all of the right things to strike up rapport, get the number and set up the date, but because you are merely just another M&M in the bowl, the effort for a woman to follow through on these things will always be at a minimal level. Women in such a mindset might flake simply because their feet hurt and they don't feel like going out, or they are just having a bad hair day... stupid little things.
Maybe all you need to do is add that little peanut into your interactions.
Personality. Passion. Chemistry. Something that can give a woman the impression that "there is something different about this guy" compared to 100 other guys who are out there doing exactly the same small talk > number > date routine you are doing.
You could be sitting here on the verge of giving up when in reality you might be so close to taking things to a new level with this one small fix.
I think that's worth pursuing.