The hotshoe on a Nikon SB800 and SB600 can be really loose and doesn’t lock in its place due to a broken pin that doesn’t pop up. I myself bought 4 of these flashes from eBay and 2 of them are in mint condition. Even then, all my flashes have a broken hotshoe.
My solution: “I simply use a rubber band and put it on the swivel and connect it on the hot shoe plate since I always use these flashes off camera not on camera.”
But I ran across the options to fix it with a small part purchase but then that link is broken and that part is not sold anymore. If you are handy and not worried about doing it yourself, I found the below post from Flickr.
“I disassembled the foot. It appears there is a white plastic piece that is broken inside the foot. This little plastic piece attaches to a rod. When you turn the lock lever, the rod turns and turns this plastic piece. That, in turn, raises or lowers the pin into the hot shoe. If this white plastic piece is simply spinning in place, the pin can’t lower into the hot shoe. I’ll try epoxy, but doubt that will hold it for long. Too bad Nikon doesn’t sell that tiny white plastic piece for 4 bucks.” from Flickr