[nirofo]

It's probable that you don't have a good solid connection with the battery terminals in the flash unit if you leave the door open, can you get power if you press down firmly on the dummy batteries to make sure you have a good contact. You need to cut a small notch in the bottom of the door to let the wire come through with the door closed, it shows you where to do this on one of the web link instructions. If you don't want to do this then you can either put something round the unit to hold the door tightly closed, or, the best way is to purchase a spare door from a Nikon service centre and adapt it for the wire to come through. Check your connections to make sure you have then correct, here's a photo showing the connections for a Nikon SB-800, they're the same for the SB-900.

nirofo.