After having sex a couple of times, I want to see the girl again and that usually never happens and she flakes. Happened so many times. This is one example (over text):

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Me: Do you want to hang out tomorrow or Monday?
Her: With how I feel, tomorrow is out maybe Monday but I'm gonna be working in xyz hopefully.
(She said she got drunk last night and today she was recovering. It will make sense if she cannot meet today. Why she can't meet tomorrow?)
Me: Ok... You can have one more night of drinking, binge drinker. Have you started on pqrst (change in topic)
Her: Not drinking for awhile .... .... ....
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Me: Ok.. So if we meet on Monday what time will it be. I will be busy in the morning
Her: I'm busy in the am so you'll have to text me then & see if we can work something out.
Me: Ok

Why is it never a sure YES? Many girls do this.. they never give a definite time, they always beat around the bush.
How can I prevent this from happening.. if not, how do I deal with this type of situation?

Because you're asking her when SHE is free and IF she is free, what time is she available?
Supposed to be on your terms. After she said she was recovering on that day. You should have replied with,
'Kay, maybe another time.' Then deleted her number and moved on to the next girl.

Women can't make decisions.

Start telling her and stop asking.

You are of low value, not high value. When you are high value, she will clear her Monday for you.

Being hung over is a good excuse, it happens. But everyone knows hangovers don't last 2 days. Your best bet is to stop asking her to do stuff, and just be casual around her. You're too busy to see her or have other girls you'd rather see. This is easiest accomplished when you actually have other girls you'd rather see, so go find some.

Two things I can suggest:

1. Stop asking them, tell them what to do

2. Find out why they don't want to meet you again. You might need to improve your bedroom skills.