I don't get much conversation on here-- but on another web site, even though my profile says "not looking," I often get a lot of chat invitations. Like, too many to be able to talk to anyone. The more attractive guys on here probably have the same problem.
When I tried in the past to politely explain that I was not interested, I usually got a nice reply, like "sorry to bother you." But too often I got responses like "why not," and people who will argue with me. When I WAS looking, before I was in a relationship, it was even harder, because if I said "you're not my type," they might say "what IS your type," or "I look better now than my picture" or even "I just want to..." etc. It gets old real fast, and at those times, if you really DO want to chat with someone on the site, but not with EVERYONE, you just have to ignore other people.
So when someone is ignoring you, just be patient. They may be talking to a bunch of people, they may be not looking, or they may not be interested in you, regardless, they are not obligated to reply to every message or chat window they get. If you try a chat, and they ignore you, send a mail message that just says you'd like to chat sometime if they are interested, and leave it at that. If they ignore the mail, you should probably move on.