I recently replaced the faucet assembly in my shower. Everything was OK when I finished, but now that I've been using it for a while the handle that switches the water from the tub to the shower tightened up. Now it doesn't move at all so that the water only comes out the shower. What would cause this? How can I fix it?

Your diverter is stuck, is it a twist or a pull up and down knob?

Twist

Are we talking about a three knob shower valve? (see image) Tom

Yes

Remove the knob and cover plate. Replace the knob and loosen the top nut until the stem turns freely. STOP! When the knob turns free. If you loosed too much you'll have a leak. Good luck, tom

Thanks I'll give that a try.

Does it matter if this is washerless or not?

Does it matter if this is washerless or not?
Do you see the top nut when you remove the handle and face plate? Tom