Showers here tend to go wrong slowly — a little stiffer, a little less predictable — and the cause is nearly always scale inside the valve.
Scale on the valve
In hard water, mineral buildup collects inside the mixing valve cartridge. It rarely fails all at once — it gets gradually stiffer and less reliable over months, until someone gets a shock when a tap runs elsewhere in the house. Replacing the cartridge restores proper control.
Blocked shower heads
A weak or uneven spray is almost always scale in the head. Soaking it in vinegar clears most of it, and if it returns within months that is your water telling you something.
You may not need to lose your tile
Worth knowing before you agree to anything. Many valve repairs can be reached through the existing trim plate, or through a small opening in the wall of the room behind — often a closet. Before accepting a quote that involves removing tile, it is entirely reasonable to ask whether the valve can be reached from behind.
A stain on the ceiling below
Usually the shower base, the drain connection, or grout letting water past. Three different repairs, and knowing which before opening anything is the difference between a small job and losing the bathroom.