Several years ago i had the fixture in a stall shower replaced. The new fixture has two valves and uses ceramic cartidges. Two years ago I noticed that the head pressure was declining. Removing the head confirmed the reduced supply flow. Now there is almost no pressure at all.
Also, it seems that since this started, that the pressure is the same regardless of valve setting, hot, cold, or both, full
open or partially open.
There is a single water supply to this bathroom and both the toilet and basin have great pressure.
There is no easy access to the fixture.. tile in front, finished wall behind.