Marginata is relatively easy to grow but does not like to stay wet. It only likes a little water. I believe overly wet soil may be your problem. Normally, you can just stick pieces of a lanky stalk into soil and they will all root and bud into new plants.

Take the cutting out of the wet soil and put it in dry soil. The cutting is probably water-logged. Do not water it. Because it has no roots, you will see it wilt, lose some of its leaves, then begin to come back to life. I hope that will be the case with this remaining piece of your beloved plant.

I happen to have some Marginata (dark green w/red edge variety) that I pruned a month ago and set aside. My plant had 7 stalks in the pot and got lanky, with lots of foliage just at the top. I wanted to balance it out with foliage at the bottom, so I pulled 3 plants out and cut them into 3 pieces each (top, bare center, bare rooted). I placed the tops and the bare center twigs into soil. They have all come back to life and budded.

I still have the 4 rooted pieces that I pulled out of the soil. They have not yet budded, but I checked them and they are still alive - there's green under the bark. I had them clumped together in a small pot with minimum soil but have been watering them. I think that's why they haven't done anything, so I just added a little soil and cut off their callosed tops. I'm certain they will bud now.

If you'd like any of these, I'd be happy to send them to you. I really don't need them. Once they grow, you can cut them down and stick them in the soil at different heights for a fuller plant. The entire stem can root. You just have to make sure you keep track which side is up and which side goes into the soil. I place them right in the soil as I cut so they don't go in upside down. They won't root that way.

Let me know if you're interested.