Do you have VSYNC turned on in the video settings? Most monitors have refresh rates of 60 Hz, & most games have a VSYNC setting that when enabled caps your FPS to match the refresh rate.
It's actually a good thing: if you're playing below 60 FPS anyway, you won't see any change; if your machine is capable of more than 60FPS, you won't have the "ultra" picture but your picture will be consistently smooth (i.e. no variations in FPS).