I have a shower/tub combo. When we turn the knob for the shower to turn on while the water is running we get nothing. Any ideas?

It looks like the filter, or flow restrictor, in your shower head is clogged up. Remove shower head, look inside the Female Threaded end and clean it. Run water through shower arm only before you reinstall shower head. Then, reinstall.

If you remove the shower head and still nothing, it is most likely the valve below.

You can take the knob off and then remove the valve with a wrench to check the rubber washer on the end. BE SURE TO TURN OF THE MAIN WATER SUPPLY before any plumbing project!

A basic hot or cold shower valve is really just a brass screw with a couple rubber washers.
If the washer on the end of the valve gets detached from the valve it will stay in position to block the water flow even with the valve screw in the open position...

Take it apart and you will see what I mean.

I suggest taking the valve with you to Home Dep*t so you can get the right replacement washers.

Remember when re-assembling a little faucet grease is good and don't go too tight or it will be hard to turn the knob.

Good Luck