How do I get more water pressure in the tub and shower? Even though old cartridge has been replaced, nothing is happening.

How do I get more water pressure in the tub and shower? Even though old cartridge has been replaced, nothing is happening.
As you did not mention that, I assume that water pressure elsewhere in the house is OK.
First close the main water valve, than open both taps, and check for blockage : there may be dirt, sand, or stone blocking the passage.
Also check any valves in the supply line for being fully open, and (as with the taps) for dirt blocking them.
Last possibility is that somewhere the pipes in the supply line have been deformed (earthquake country?)
Success!
;)

How do I get more water pressure in the tub and shower? Even though old cartridge has been replaced, nothing is happening.
A reaction to an answer is always much appreciated, even if it dose not solve your specific problem...
:)