I'm going to take a swag at this for the first pass because the following is most common cause.
New Posts is geared toward your and each individual member's profile. What I mean is the software senses when you sign in and shows you "new to you" posts since last sign in.
So if you're in and surf, and check new later on subsequent sign in, or if you don't sign out. You're new post is now old.
Could this be it?
It showed up some 2 hours later in New Journal Posts, but did not in "New or Today's Posts