If it is not electrical, then it looks like the adjustment of the actuating rods for the legs. You have to adjust their lengths properly so the internal springs have the correct pre-load which ensures a positive lock for each leg when it reaches the down position. I haven't got my maintenance manual with me, but I think it is a minimum of 0.25ins of residual stroke left when the legs are in the locked down position (check the manual). A big tip when checking the gear operation with the aircraft on jacks is to always apply the specified load on the nose leg to simulate air loads, I use a length of wire with a spring balance...you need a helper to do do that!