Don't take that sort of thing personally in any way...I'm sure a lot of gals experience the same thing.  BTW--it can be equally frustrating for guys who make an inquiry & receive no respone from the gal.  

Some gals simply be too busy to reply to everyone who emails, & some guys may well just "cast their hook" & ignore all but those who respond immediately.  Another thing that can enter into the scenario is that someone might not feel comfortable in handling a "no thanks" situation.  It's much easier to simply hit the delete button.  Nah...it shouldn't be this way in a perfect world--but since when did the world get perfect?  Just keep it all in perspective.