The pilot carried out the various pre-landing actions, and carried out a normal approach. Shortly after the aircraft touched down, the left wing began to drop. The pilot immediately applied full power and went around. As the aircraft climbed away, the pilot of another aircraft observed that the left maingear had failed to extend. The landing gear was cycled, and again indicated a safe condition. During the subsequent landing roll, the left gear leg collapsed. Investigation revealed that the left gear uplock roller was seizing intermittently due to insufficient lubrication. This caused the gear to jam against the uplock when the extension cycle commenced. The retract rod subsequently became bent, preventing the gear from being fully extended. In this aircraft type, the gear position microswitches can give a down and locked indication even when one or more gear legs is not in the locked position.