: thanks in advance.m

If it is a two handle valve it is most often debri in the diverter of the faucet. This plastic diverter is located in the cross of the faucet from which the tub piping comes down and the shower riser comes out the top.
You can: l. try to blow the debri back out with air. Turn off the water, Remove the stems, cap off the shower arm, remove spout and install an air connection. This does not always work. I usually have to,
2. Remove the faucet from the wall and access the diverter from the top. It's a two to three hour job and costs about the same as replacing the faucet. $150.00 labor.
Lonny the Plumber