try this...
Tools>Options>Rendering tab>
Quality Shaded
Show back facing polys ON
Every patch has a direction ("normal") and if it's facing away from the camera it won't show up in shaded mode if "Show back facing polys" is OFF.
You can also flip a patch(es) by selecting it, right clicking and choosing "Flip normals"
normal direction is important for particle effects, porcelain material and five-pointers, which must match the patches around them.