Upload & Sell: On
Check the autofocus settings on your camera body. On my 5D3/1DX, there is a setting called "Lens drive when AF impossible"- or something close to that. Basically, it is designed to prevent excessive lens drive as the lens hunts for focus. If this setting is enabled, you might have to manually turn the focus ring to a point that's almost in focus before the lens will attempt to AF.
That's the first thing I'd check- assuming the lens switch is set to AF, of course. Also wouldn't hurt to make sure you haven't set the focus limiter to a range that's outside of the subject you're trying to acquire.