I am assuming you are reading a topic, and click on the PM icon under a users information. This brings you to your PM box with a new PM to that user. When you hit the submit button, the PM is sent to that user and doesn't post a message to the topic you were in. Not really sure what you are doing, but thinking your doing something your thinking is something else.

From a topic, if you select the reply button (top/bottom of the topic) or the Quote button (on each individual post), then it will reply to the topic (that and the quick reply at the bottom of a topic replies to the topic.
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Charles Gallant