How can I get my shower to work correctly?

No matter how many water descalers you fit the valve will be clogged or if not the valve the shower head you need to dismantle the valve and clean it out check for inline filters as well. you will find these are probably gunged up if this fails you could be looking at a new cartridge if the shower mixer is type with replacable cartridge also check any isolation valves which should be fitted .They may be clogged

Steve the plumber
October 2008