I'm not familiar with that particular breed of bantam, but our Serama Old English crossbreeds are excellent fliers. One of them keeps trying to lay her eggs on top of the 6 foot tall run.
I wonder, though, from your description whether the setup you have makes it difficult to get into the nestbox. Does the hen have to get up to the roost then get down from that to get into the nestbox? Chickens aren't helicopters and can't really go straight up and down. You have to allow enough horizontal space for them to maneuver. Do you have other hens that are able to use this box?