Hi, Steve! I can tell you - TAKE IT EASY!!!
I don't think that 'not respond' is the matter of politeness or impoliteness. If a man receives 200 letters a day, he has to kill the night to reply or refuse politely all of them. Women have the same situation. It's like a hypermarket - a big choice, the unnecesary is near but the thing needed takes time to find, and sometimes it seems you have found - you're looking at it carefully, reading the lable, touching and suddenly realise that it's not the right thing you are searching for. I think people should read the profiles carefully and understand if he/she corresponds to the requirements (age, country, education and so on) and then tries to contact.
In my opinion, the most common reason for 'no reply' and 'unexpected stop of a conversation' is the absence or very little interest in the person. But you shouldn't despair - the door will open if you knock, even if you have to knock at millions of doors in this maze. If you really like someone, be persistent!!!