I created a solid part with some complex surfacing. When trying to shell it, Pro only lets me make it very thin. What can I do to increase the shell thickness.
Without having the part it's hard to tell. One problem that often prevents you from creating a thicker shell is that a radius of some kind disappears because it is smaller than the thickness you desire. Small surfaces that disappear often cause these failures. You may be able to fix some small surface problems by using control points when creating the surfaces and eliminating this areas. There are many techniques used for cleaning up surfaces.