If you can not get the lock to lock there are a few things to check. First take the U portion out of the crossbar - does the key turn a full 180 degrees? When you put the U portion (shackle) back in the crossbar does the key still turn a full 180 degrees? If not, you need to check the spline which is the silver piece on the U portion of the lock. Sometimes the screw holding the spline in place will loosen and it will slip down toward the "pointy" end of the U. This will cause the crossbar not to "sit" correctly on the U portion. The U does not go far enough in to the crossbar for the locking mechanism to "grab and secure it" keeping the lock locked. The spline should be located directly against the black vinyl covering the U portion of the lock. Located under the spline there are rubber bumpers the will rest on the crossbar when the lock is in the closed position. These too can ship down the U portion through normal usage. The bumpers should be positioned touching the bottom of the spline. See Below.