I am recently experiencing low Hot water pressure in my uspstairs shower only. What could be the problem? And will cost a lot to correct?

I am recently experiencing low Hot water pressure in my uspstairs shower only. What could be the problem? And will cost alot to correct?
If you're handy it should cost nothing. Shut the water off to the valve and open it up. Check and clear the inlet ports in the cartridge. Now, before you reassemble, turn the hot water on for a minute to flush out the supply. Reassemble and test. Good luck, Tom

I did this and i got a lot of build up coming out but pressure is still low. What suggestions do you have next?