question that requires more knowledge than logic?

A scientific answer would require me to have an in-depth understanding of your entire life. Studies would have to be done to determine exactly what makes you "tick." If you want a scientific answer, you should probably visit a doctor or medical website. No one here can psychologically evaluate you.

You are probably just nervous around girl in whom you have a sexual interest. You are afraid of saying some inappropriate or controversial thus fearing their loss of interest in you.

The ones you are not shy with are ones you have no interest in screwing. Thus, you have nothing to lose by saying what you want.

But you can't guard your words all the time when you are interested in a woman. Maybe you can fake it for a few dates, but eventually you'll revert to being you. You might as well be you up-front.

Some girls could be much sexier then other girls so you talk to a plan girl and your fine you talk to a sexy girl and you might have a hard time talking to her because she is more attractive

Do any of the girls you have trouble talking to have anything in common like clothing or looks or personality look of key factors like that so you yourself can understand what is wrong

It could be the girl, and not you. Some people's really outgoing personality make other people shy around them. But if there is a girl who's laid back and the quiet type, you would feel it easier to talk to her.

Usually I'm more shy with guys I really like.