Sometimes changing up the order of extrusion worked for me. Extrude the center first and the 0 next. That sort of thing.
Try hiding everything else and extruding the center.
Try extruding to a different height. Then adjusting later.
All these things worked for me in the past. 
Made many of these and those are the things I had to do to get them to work. I do not know why. I am assuming you placed the sketch of the “0” in the circle clock face.