Others have also asked a similar question and when given the possible reasons why their messages do not receive a reply, have understood.
1. The person you send a message to may not actually be online though it may show they are. If you don't click the 'logout' button, the system assumes you're still active.
2. It depends what you say to them. If you contact a girl with something along the lines of, "I'm hung like a donkey so how about it, darlin?" will only be met with laughter. Just because this is a sex site, it doesn't mean your manners should be left at the door.
3. Those you contact may not be full members so are unable to reply.
4. If you don't satisfy their criteria, you may well be ignored. If a profile states, "No one over 50" or "No men" yet you decide to contact her anyway, why are you surprised at not receiving a repsponse? Unfortunately, a vast number of men on here don't read a profile before sending a message and then complain they don't get an answer. My own profile asks people not to use any text chat, but many do, which is why I don't reply. Respect what a profile requests. If you can't do that, don't make contact!
5. Maybe you're just messaging women devoid of their own manners.