Too many of the women on the site are too selective. They don't even bother to respond or sometime even check out my profile, although I have had one or two respond to say we weren't a match or to reply to an anecdote I gave after reading her profile.
I paid around $14 for the month in order to be 'invisible' and see if my messages were actually being read. (They were). Unfortunately, as aforementioned, the response rate is horrible.