The commercial pilot stated that he locked the tailwheel on his single-engine airplane prior to departure. After takeoff, he stayed in the traffic pattern and landed. However, during the landing roll out, the airplane ground looped and the airplane's lower right wing contacted the ground and broke the wood spar. The pilot stated that on examining the airplane after the accident, that either hardened grease and dirt build up may have prevented the tail wheel locking mechanism from fully engaging or that the mechanism may have been inadvertently bumped in flight.