SHEESH........the solution lies in your timing, her availability and your ability to write a enticement on the fly..THATS IT.
DAVE M has a 2 part email system designed to evoke a response......if she still dont respond then he sends out the WHAT GIVES EMAIL or text message.
Nexting doesn't always work.....like you said, tons a variables. Some chics just got too many emails/IM's to sort thru. But........I have gotten thru from timing and subtle persistance after what any normal guy would have nexted.
