I think it just takes a long time. I also have a theory that only watering them when they are quite dry may also help to get bark in the trunk. It certainly keeps the internodes tighter. I read that these trees can expand by as much as 15% when dry and then watered. I’m thinking that routinely putting them through that cycle helps the trunk get wrinkled looking much like a cork bark dwarf jade. Anyhow, that’s my idea.