They run because there is something worng with the guy (ie looks, he said the wrong thing during their date, flirted unknowinlgy, ate like a pig, burped, interrupted her while she talked, your sense of fashion is weird, you cannot stop talking about yourself etc etc etc) and they think they can get someone better or they are not ready for committment. She is not that into you.
Usually an educated woman won't tell you why she does not want you because she is being polite. Anyway maybe the guy she has been interested in all along came back and he now wants her.
Most of the time the woman is not that into the man if she is not returning the phonecalls after a "great date" and I am speaking from experience. Even the voice of a man can be a turn off.