Aaaand that's the problem: your response. There was a time where I'd say the same thing, and get the same results you're getting now. Now, if I get this response I say "I'd be easier for me to know now so I can plan out the other parts of my day/week."
Now, you may think phrasing it this way is forcing them to give me an answer on the spot - and it is. The reason I do this is because I have found through my years of dating that (a) a woman who has a strong interest in you will say "yes" to the date and rearrange everything else to make sure she can show up, and (b) most women already know their schedule in advance, and know in the moment the days that would work best for them. If a girl has a regular 9 to 5 job that she goes to every day, chances are she can find an evening where she has an hour to meet up for a coffee shop first date. So, more often than not, if you're getting these responses, it's because the girl has low interest.
With that said: you may also want to see if the way you're asking them out is causing your negative results. Are you asking them out like "so, are you free tuesday at 8?" or are you saying "so, let's go out for coffee; i'm free in the evening on tuesday or thursday, which works best for you?"
What's the difference? The first way is giving her a choice between something and nothing, whereas the second way is giving her a choice of two days to choose from, i.e. between something and something. People are psychologically designed to make a choice between two things, so you always want to make sure you're presenting something and something vs. something and nothing to better your chances at having your request be a "yes" on the spot.
Anyway... hope this helps!