I saw posts on your site about very low pressure from tub faucet and shower head. I have the same problem in a house built approx 1981. You mention a "cartridge" - what is that? Also, if it is a buildup of calcium deposits or other blockage, is there a special "snake" that can safely be used in the line without pipe breakage? All my other plbg works fine, the problem is only in one of my tubs.

If this is a "two handle" tub/shower valve, then the problem is most likely a buildup around the restrictor in the body. If so, the only solution is to unscrew the shower riser and the tub spout connections, then "drive" the restrictor out, and then connect the shower back with a "twin elbow" and 3/4" diverter spout.

I hate when that happens. You might begin with working backwards. Remove the showerhead and turn on the water and measure the flow.. or lack of. Soaking the showerhead in white vinegar, overnite will clean out most mineral barnacles. Check the supply pipe coming out of the wall for any mineral buildup. The cartridge, is usually associated with single handle mixers. By turning the mixing handle, to and fro makes the temps change from hot to warm to cold. Cartridges are not universal. Its important to know the brand of shower assembly you have. A picture posted here will also help us to help you.....
Best Wishes