We bought a house for our daughter and in both bathrooms the shower diverter is not working. It is the type that turns from left to right but actually it now turns all the way around. On one of the two you can get it to work if you tweak it just right, but the other isn't working at all. I do not know the faucet or valve maker but it looks like a pretty standard set-up. If it helps, the house was built in 1981.

Remove the knob and face plate from the diverter, (see image). On the diverter that turns all the way around it sounds like one of two things. The knob/spline is stripped or the threads on the stem's stripped. The other sounds like the washer needs to be replaced. You will know better once you open them up. You shouldn't have to shut the water off as long as you don't turn on the valve when the diverter stem and washer assembly is out of the valve body. Good luck. Tom