Zoom stopped working

The double-tap with 3 fingers action to zoom stopped working on my iPhone 4s yesterday. This feature is turned on, and I've tried turning it off and one and shutting down and restarting my phone restarted my phone twice with no change. When I double tap with 3 fingers I get about I see a slight movement in the screen like it is trying to zoom, but does so with only the slighest fraction (almost not noticable) of zoom. A repeat dbl-tap w/ 3 finges sets it back to original scrren size. Ideas?