First you said decent rapport, then you said your conversation lacks luster/is flat and awkward. I would say that if you had better connection the results would be different. When I say connection I mean something different from rapport. Something you have in common, something deeper than the natural tendency of the body to become in sync with another body.

Then you go back to your friends and she goes back to her friends, meaning you don't feel like spending more time with each other.

4 tips:
1. find something in common with her for a real connection, not just rapport
2. when you go back to your friends, say something like "What are you doing later tonight? I (really) feel like spending more time with you NOW, but maybe it would be rude to your friends to steal you, maybe talk more tomorrow or later tonight?"
3. make eye contact with her a few times EVEN AFTER you #close and/or kclose. You can be sure she is trying to make eye contact with you. If she isn't, go over there on your way to the bar or the bathroom and touch her slightly to indicate that she is more than just a kiss and a random number, let her know you're still thinking about her
4. don't talk to other girls in the bar except the ones in your friend circle

If you have trouble creating a real connection with someone, try letting your guard down just a little bit, let her know something about the real you, maybe not lie about your age? That just shows you're insecure and manipulative, real people don't want that kind of person around.