Thats a known issue, dont know if it is intended by Funcom.
You simply have to take care that the Thrall who is bound is still agile.
A little bit hard to describe but let me try it this way.
If you knock out a Thrall and “immediately” bound him and dragging him home all will be fine with the elevator because he ISNT stiff. (flutter behind you while dragging)
BUT
If you knock out a thrall and move away to far and came back to the knocked out corpse, you will see it is stiff with outstretched arms (deathly rigid).
If you drag the thrall now, you will see, the thrall isnt flutter around anymore.
These stiff guys immediately drop and break the rope after you try to leave the elevator on the top, fall and die.
I figured that out and reproduce it many many times with allways the same result.
So take care that the dragged Thralls are not stiff (deathly rigid) and all is fine by using the elevator.
If you knock out a Thrall, dont go to far away or it get stiff.
Hope this helps to understand why some thralls work with the elevator and some do not.