Do as Bill suggests.
Here are the adjustment procedures. Rebuilt shifter boxes are available.
Unfortunately, if that does not fix It, there are a variety of other things that contribute to cause this problem. These are lower probability problems: worn or missing parts like shifter forks/levers, detent balls, and synchronizers (I have been told - I know nothing about the insides of transmissions).
I had finally gave up and switched to an SM420 4-speed.
I got a few extra years out of my 3-speed (after getting a rebuilt shifter box) by being careful to never shift from 3rd directly to 1st - shift (with the clutch depressed) from 3rd to 2nd and then from 2nd to 1st.
Good luck.
