I'm not sure, but I think that the posts in "New Posts" only show up there if there has been activity on the thread since you last viewed it. It may also be that posting to the thread is considered viewing it, so if you posted to the thread and then went and looked at "New Posts" the thread wouldn't show up then.
The only way I know around this is to go to the "Forums" link and look for it in the forum where you posted it. Or wait for someone else to post something to the thread.
"At times the world can seem an unfriendly and sinister place, but believe us when we say there is much more good in it than bad. All you have to do is look hard enough, and what might seem to be a series of unfortunate events, may in fact be the first steps of a journey." Violet Baudelaire.