Phil, I'm doing it exactly as you have it outlined. (I just tried again using your instructions).

Load David, inject the character, (making sure that body is selected) hit the Full Body Morph and then save the figure.
When I bring the figure back in, only the head morph is saved as the FBM. (Setting the body morph to 1 changes the head back to default David and leaves the body as is.)
And when I ask WW to Analyze Full Body Morphs it gives me a list that includes the FBM I saved.
If I select only the FBM, it completes in about 1 second and resets the figure to the default David.

I haven't tried V3 or V4, but this is what is happening with both David and M3.  I've tried it with characters made by several different merchants as well as ones I've made. Same results every time.