This is gonna sound like tough love but you gotta understand that its coming from the right place.
There is no such thing as flaking when it comes to FBs.
Look at it this way: What if your male buddies started doing this stuff?
Would you care at all?
I sure wouldn't.
Looks like you are tho, just your two posts here reveal NRE. Probably because she's a good looking 20 year old and you maybe don't get with those types too often.
Also...she's a 20 year old chick. Chicks that age generally have no concept of following through on plans cuz they simply have no incentive to.
I'll be honest dude, you should probably adjust your expectations of this chick especially if she's just an FB.
I mean if I have an FB who is meh looking and pushing 50 and I expect her to flake/get a bf, what should you be doing?
Do I leave her alone for a while
Yes. Literally stop all contact and focus your energy on getting another FB. Let her hit you up, if she does. Considering she's 20, she's probably already forgotten about you. That's how chicks under 30 are mane. Just gotta suck it up.
Again, there is no such thing as flaking when it comes to FBs. If they fail to meet up, go get yourself another FB who will agree to hang out with you. Simple. And to me, there isn't such a thing as flaking at all tbh. If any chick fails to meet up with me then big whoop, I'll just find myself another chick no matter how many times I have seen her or how many times we have had sex.
This is why it is important to have a damn rotation. Just like you should be diversifying your income (a hard lesson that I've been learning over the last three years) you need to diversify the chicks that you are seeing.
For example, I got two chicks I'm trying to lock in and an FB. The FB recently got a bf but dumped him cuz she thought he was running around (he probably wasn't but she gets real paranoid when she catches feels). I only see her if I haven't had sex in a while. The other two are single. I'm very close to locking in one and its only a matter of time before I lock in another. I plan to make one an FB and the other a kinda serious relationship. Either way, it isn't enough and I need to add a couple more to my rotation since chicks come and go constantly these day.
Unless you are trying to start a family or something, you should always be going out with about 5 or so chicks at a time. Keep a rotation and you won't be posting topics like this anymore. Make it a new years resolution.
Hope this helps.