It's seems your airplane is sitting a bit below ground when editing (this is the default slew position, which means the airplane weight is not taken into account) and it's likely very light loaded, so it's very high on the shock absorbers.
I see your back door is 4.25 mt in height, when editing. The highest the GSX staircase can reach is 5.05 mt so, if the actual height of the back door of the airplane in normal usage (when airplane load IS taken into account) is higher than that, the stairs will be positioned a bit back, but the stairs can't reach it because your door is now higher.
I'm not sure if this is realistic for a 747, that depends how the airplane developers modeled heights, weight and shock absorbers effect but, you should be able to fix this (at least a reasonable fix) by LOWERING the doors height in the GSX editor a bit, even if not perfect, it will be better than not being able to connect.
How course, this will change with the airplane load so, I guess that if you load it more, it will sit lower on ground and the stairs might be able to reach it even now.
