No problem here with my D4 and 400/2.8. Just used them both today. Can't imagine why it would not dismount, unless the mount on the body or lens was loose in the first place. The Nikon mount is pretty fool proof. Have used Nikons over the years since the Nikon F, along with Leica M & R, and for a short time Canon. The only mount that ever gave me trouble is the Canon mount.