Usually when chickens don't go into their house it is because of a mite infestation. Having said that welsummers and leg bars are the lighter more flightier breeds so they will have a tendency to perch high. I had some friesian fowl who lived in the trees through winter. Drove me mad with worry, but they survived. Obviously they lost condition.
If you can't see any mite and are absolutely sure there arent any in there, then I would be tempted to put some posts up and give them restricted range and net above. Your chickens are young and perhaps need to learn where home is. Alternatively, poss net across from tree to tree? Hope this helps.