ZSketch - Maintain Volume
How to keep the end Spheres from collapsing?
Create zsphere armature that extends into and fills up those end pieces and the voxelization that takes place when you make a unified skin will have an easier time working out what it needs to do.
Simply take the zsphere that you started with as your base to zsketch over, then go and add more zspheres to it carefully and strategically moving them and extending them out into the areas that need some support, placing them inside the areas getting lost in the process.
This doesn’t always work however when done after the fact like this. Sometimes it does and it’s easy enough to try. Sometimes you just either have to restart a new sketch based off a new armature of zspheres that account for those problem areas beforehand (zsketching isn’t as freeform as we think it is, the skinning process still needs a logical support most of the time or you’ll run into this problem here all the time until you learn to expect it and compensate for it while working). Worth a try though, sometimes some little zsphere support under the extended and thin areas is enough for the unified skin function to properly generate a skin. If it works, you can have fixed this problem using this method in less than the time it took me to type this 