If someone doesn't reply to a PM you sent then, odds are that it's simply because the PM didn't catch their attention. Some people get tons of request upon making a thread, and they kind of have to 'filter' the good ones out. So odds are that your PM simply didn't pass the filtering process, which would indicate that you need to put some more effort into it next time. I personally try to respond to every PM that makes at LEAST a bit of effort and provides me with more than a few lines, but a single line? I can easily ignore those, and so would most people, especially if that person put a lot of effort into their request thread.
Do you have any idea how irritating it is to spend a few hours working on a request thread, putting lots of effort into it, fixing every little detail and adding all the little quirks... Only to be sent a PM like ''I want to roleplay with you.''? (And here I'm using good grammar, most people don't.)
So yeah, simply put, odds are you just didn't get a response because you didn't put any effort into your PM, which honestly, some people might see as downright insulting, if not a sign of how boring their potential roleplay could be.
Just a word of advice.