If it's not locking it's probably missing the locking locking spring.
The locking spring is right under the black plate beneath the RW knob. The plate is held in place
with a "C" clip. When that's removed, the plate can be lifted off and you should see two springs, one coil spring and a spring that does the actual locking looks a lot like a hairpin and is at about 12-1 o'clock.