OK, so you can have a morph if there is a gap - so no problem seeing a morph.
Go to the selection tool and make sure the 'quick select' magnet is active.
Also choose the 'select sub-elements' option.
Now click on the side surface to select it.
Now drag from the bottom node and choose the tube command again.
Drag to close the morph (click twice on that close point).
Does the morph stay or disappear?
If it disappears, I have no idea why.
If it stays, I have no idea why this is any different to you tubing all the way around in the first place.
But at least you have your morph.
Don't forget to set your selection arrow back to normal (not sub-elements) or you will have trouble selecting things.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10