Can anyone tell me why my attempt at an air layer on this hornbeam has not been successful? The tree refuses to grow roots but instead grows gnarls to reconnect to the lower section. I have repeated the attempt twice since since spring in the same location with no luck. Both the upper and lower part of the tree are healthy and have lots of new growth, except roots!

The substrate could be too wet and dense?

I have found hornbeam to root slowly in layers, and I have had to re-cut the callus. I assume you have sliced the callus & added rooting hormones?

Looks like you did not remove (good enough) the whole cambium layer. In that case the tree could made some bridges of callus over the incision. See arrows.

Hi, yes this is the 3rd set of calus and I have repeatedly cut and used hormone powder. I think maybe you are right and the moss is providing to easy source of fluid and nutrients so it doesn't feel the need to root?

Thank you for the response, I will try again now as I still hope there is enough time in the season to establish some roots. I was so sure the 1st attempt had been a very clean layer and this photo is after the second failed attempt. Maybe I will try to be a little more persuasive this time!
I will keep you all posted, I'm hoping it works as the bottom and mid section of this tree both have great potential as bonsai.