Do you have a marquee in 2D and are showing only the marquee region in 3D?
If so, it is possible to draw a morph "out in space" beyond the invisible marquee region... and it will not show in 3D. Either show all in 3D, or make sure that you snap your Morph to something visible?
One of the forum moderators • AC 25 USA and earlier • MacOS 11.6.5, iMac Pro