At the beginning stages of a relationship, there really is alot of power struggles going on. It sounds like from what I hear they lose interest because you are calling them all the time. If you meet a girl that you really like, in a couple of weeks you should have maybe had 2 phone conversations and hung out with them 2 or 3 times max. Anything more than this and you're coming on too strong.
If you hold a tight frame, the will start calling you and asking you to hang out, then you have the confidence to attract them even more.
Check this:
You meet a girl, your like her, she likes to you. you get her number.
Scenario 1: You guys go out, have fun, you call her to say you had fun, you call her to wish her a good night, whatever, and then one time you call and she doesn't call you back. Now your panic, you wonder, o-sh1t, what happened. You like her more but don't know why (cause you don't have the power) Now you are thinking about her 24/7 The next time you call you sound more nervous, more edge, more needy cause it's been on your brain, There is nothing that is fun and interesting about you.. this is a downward spiral which is very hard to reverse once they start losing interest and yours keeps rising..
Scenario 2: You guys go out, have fun. Then you chill. You chill for like a week and don't call her, if she calls you great, now when you talk to her, you are your normal self and even a little more confident because this chick likes you or she wouldn't call. You wait a week to ask her out, but maybe she asks you out instead, now you have even more fun cause you really know she likes you. The trick to this is to not let yourself get out of your frame till they are begging for whatever you desire, sex, relationship, I love you, marriage, whatever..
Bottom line is once attraction is sparked, if you try and push it too quick, it will burn out. If you take your time, it will continue to grow as long as you stay interesting. Remember Fellas, it's always easier if she thinks she likes you more than you like her.