I'm not a Don Juan but I do know a lot about rejection.
A lot of the times, when a woman says she's "too busy", she's bull****ting you. Many women are indirect when rejecting a guy; that's why you'll rarely hear them say "no" or "i'm not interested". And instead, they will string you along. They'll show enough interest to keep you around but if you dare ask them out, they'll always come up with excuses such as being too busy, or they'll try to bring friends along, or they'll take weeks to respond to your texts. Trust me; I know all about this type of treatment. I used to waste months of my time chasing a girl who honestly didn't care about me.
As men, we need to do a better job of decoding a woman's interest level. Instead of looking for direct communication, I think it's better to read between the lines and look for many different signs that a woman is giving you.
For instance, taking days to respond, always being too busy to go out but you see her posting on her Instagram where she's out with her friends, not dressing up when you two meet up, talking about other guys she's interested in or sleeping with. All of these point to low interest and if you still asked a girl like this out, it's probably going to be a no 95% of the time.
My advice would be that if a woman is continuously giving you signs of low interest, don't ask her out and/or move on. You're just continuing to waste your emotional energy into a girl who probably will never see as more than a friend or an acquaintance.