Agreeing to go for a coffee or drink doesn't mean a girl is romantically interested in you.
The older I have got the more I have come to believe in this rule of thumb:
If you sense a girl likes you, she probably does, at least to some degree.
If you fancy a girl but deep down don't sense she likes you then she probably doesn't, no matter how much you hope and wish it.
People talk about all these tactics of do this to get a girl to like you, treat her mean, don't try and be her friend else she'll stop fancying you, or whatever.....the basic truth is that is a girl like you then it is just easy, if you like her you will naturally come together. She will like being around you and feel happier being around you so its easy to get her to spend time with you, she is not making excuses not to come or to get away early.
I think if ever you have to think too hard/wonder too hard, or work too hard to get a girl then you have missed the point that she probably isn't interested, and you can't make a girl interested in you just by bidding for her attention.
Not every girl that likes you is going to get with you, sometimes circumstances aren't right, she might be with someone else, you might be with someone else, you might be in a situation like her flatmate or her manager at work where she doesn't feel comfortable entering in to a relationship. But you can always sense when there is some kind of vibe there between you.
The other point I'd make is there's a difference between 'chemistry' and 'compatibility'. Sometimes there is chemistry between you and a girl but not a lot more, and you get off with each other/sleep together, maybe even start a relationship together but it isn't going to work properly. When you're compatible then you just kind of click naturally and enjoy spending time together. There's a more outwardly obvious vibe when there's chemistry, and other people pick up on it too. When there's compatibility you just feel yourself drawn towards the person and comfortable with them. For a relationship to really work you need this compatibility and when you've got it you don't need the other kind of chemistry because you just feel as one sexually as well.