The Pilot of a Waco YPF-7 reported losing control on landing when the tailwheel became unlocked.

Narrative

A clear; sunny day with calm winds. I flew a Waco YPF-7 on a local flight around the pattern. I did one takeoff and one landing. The aircraft is equipped with a locking tailwheel which I locked prior to takeoff. Somehow it became unlocked on landing. This caused a lack of directional control and in the course of a ground loop; the right main wheel dug into soft ground next to the runway edge. Since the airplane was still pivoting quickly and since there is some bank angle next to the runway; the right lower wing and elevator contacted the ground and the right lower wing broke about two feet inboard of the wing tip. The elevator right side was slightly damaged as well. There were no injuries. I taxied the airplane under its own power back to its hangar and secured it. The tailwheel may have had old caked and hardened grease or other foreign matter which may have prevented the locking mechanism from properly engaging. Another possibility is that the release mechanism may have been inadvertently activated in flight; thus releasing the locking device which was then unlocked and ineffective upon landing. This sort of locking tailwheel is an indispensable part of airplane control of aircraft so equipped.