Why might a child with a trach not breathe past the trach (very small) when a bronch shows gd airway?

Some possibilities. If a young child's airway looks clear on bronchoscopy, but he has to breathe through a small tracheostomy, there may be other reasons why air may not flow well through the trach. Maybe there is a soft spot in the airway that caves in when inhaling (but not with mechanical ventilation under anesthesia). Maybe air turbulence and "flow dynamics" make the air not want to go through the trach hole.
Many possibilities. The trach tube might be taking up too much of the airway to breath around it. Collapse of the trachea or the larynx, or the back of the throat. Big tonsils and adenoids. Tongue base collapse.