I had a guest try to use the shower in my spare bathroom. The water turns on but when you pull down the diverter nothing happens. You can here water running for a few seconds, then nothing. If I force the diverter back up the water comes out the tub spout, but the shower just won't work. The diverter is on the spout. You pull down a ring where the water comes out. I've already tried switching the entire spout with one that works properly from another bathroom. It didn't work.

the diverter is on the spout. You pull down a ring where the water comes out.
Growler has one solution, Mass plumber has another. Let's see where the problem lies. Remove the shower head and try the shower again. If you get water out of the shower arm then Massplumber's bang on and follow growlers advice to soak the shower head or replace it. If no water comes out vof the shower are then growler's correct about the shower raiser being clogged and you need to check it out and clear it. Good luck, Tom

Since the diverter is on the tub spout I do not think problem is with the shower valve...
Sounds a little odd here, but I am thinking that maybe your shower head is clogged. They have these water saver washers installed in almost all shower heads today... have for quite awhile... very fine holes... loosen shower head and look into threaded area... what do you see..? Easy to clear if this is problem... just rinse under water for a few and will be good as new. Please let me know if this was helpful by rating my answer by clicking on the RATE THE ANSWER! Button below... thank you.

I had a guest try to use the shower in my spare bathroom. The water turns on but when you pull down the diverter nothing happens. You can here water running for a few seconds, then nothing. If I force the diverter back up the water comes out the tub spout, but the shower just won't work. The diverter is on the spout. You pull down a ring where the water comes out. I've already tried switching the entire spout with one that works properly from another bathroom. It didn't work.

It sounds like the venturi inside the valve body that goes up to the shower is plugged.

If the bath/shower has a galvanized shower riser between the valve body and the shower head and it isn't used very often, water sitting at the bottom of the shower riser can rust the opening shut.

When I come across this, I turn the water off to the valve, remove the cartridge/core from the valve body, remove the shower head and then hook up an air compressor to the shower arm and blast air through it, this usually clears the blockage.

However -- Before you go to all this trouble, remove the shower head turn on the valve and try diverting water at the spout again; If you have water coming out of the shower arm, then the shower head is plugged.

If it's plugged, discard and replace or soak in a bowl of CLR for about 24 hours.

Good call iamgrowler... *patting on back*! I honestly have not run into this before... very cool! Nice that even after 26 years I can still learn something!