Try this.
create a PCB library from the PCB and then in the library editor verify if the 3D body is there.
you can switch between 2D and 3D by using the shortcut keys 2 and 3.
AHA! I think you have helped me identify the source of the problem! I have always been working off a self-made Integrated Library that consists of three files:
.SchLib, .PcbLib, .PCB3DLib
Many parts in this IntLib (now called LibPkg?) were created back with Altium 6.X. At this time, the method of linking in STEP models was to import them into the .PCB3DLib file and then link that model to your part by adding it to the SchLib part ("Add Footprint... PCB3D"). Or at least that was the method that I used back then, and it worked. Presently, when I select these older parts under SCH Library panel, it shows that both the 2D footprint from PcbLib and the 3D model from PCB3DLib are attached to the .SchLib part. But when I add these parts to a schematic (and update the related PCB), the 3D STEP model never shows up (unless I do a Legacy 3D View). However, when I'm editing the part's footprint in PcbLib, I of course see nothing when I press '3', because there is no 3D body inside this 2D footprint.
NOW... if I take the approach you alluded to above, and I stop using PCB3DLib for my STEP models, and instead import the 3D STEP model directly into the 2D Footprint that is in PcbLib (by "Place... 3D Body"), then I can quite easily switch between '2' and '3' in the library editor. And furthermore, when I create a new SchLib part and add the 2D/3D "footprint" from the .PcbLib, then the 3D model appears in the 3D view when I place this part into new PCB projects!
So problem solved--thanks! I guess I just didn't realize that the new workflow is that you embed your step model directly in the 2D footprint. Or maybe that's always been the workflow and I didn't know the proper method. At this point, I really don't know what the purpose of having a separate PCB3DLib file inside a LibPkg would be?? I guess it's just there for legacy support??
Thanks!