It's all working fine using fujinon lenses. But I've got an old Nikkor manual lens that I attach via an adapter. I've used it many times before, but for some reason it won't shoot anymore. When I depress the shutter button nothing happens. I've set it to manual focus on the front of the body. Tried the shutter dial on 'A' and also a set shutter speed. Still won't shoot. Any ideas what I'm doing wrong?