Some apps (like Maps when it's navigating you) keep the screen on. Look to see what you're running, and figure out whether it makes sense for an app like that to keep the screen running. (A video player, for example.)
Mine does the same thing at times. Typically there is a 2 stage process in which the display dims a little & then shuts off completely shortly after. At times it will do the initial dimming but will not shut off completely. When that happens I can press the power button & the display will shut off....but just won't do it on it's own until I reboot the phone. This doesn't happen often....but just enough to be annoying....lol.