I'm no equipment expert but something doesn't look quite right with that hardware arrangement. My SBB does not provide any opening to the back, it's all closed off on 3 sides where the box sits on top, and the front entrance is the open area. And that entrance is what is controlled with or without an entrance reducer.
I've got some really old equipment given to us, that a back corner has been chewed away, and the bees use that as well as the front. I've replaced other equipment that had gaps, and the bees went to the area where there had been a hole for days afterward. I had taken away their "shortcut".