The two symptoms that are not repairs
Worth putting first, because they get treated as cosmetic and they are the expensive ones.
Tile that sounds hollow when tapped. Go around the enclosure knocking gently. A solid note means the tile is bonded to its substrate; a hollow one means it has come away, and water is usually why. This is not fixed by re-grouting — the grout is not what is holding the water out.
Damp, staining or a musty smell on the other side of the shower wall, or on the ceiling below. That is water that has already passed the waterproofing and is in the structure.
In both cases the tile and grout are the visible surface, not the barrier. Behind them there should be a waterproof membrane or backer board, and if water is getting past, resealing the surface just hides it while the substrate keeps getting wet.
That is a rebuild of the enclosure rather than a plumbing repair, and delaying makes it larger — rot spreads through subfloor and framing far beyond the visibly wet area.
Worth knowing
Sealant and grout are maintenance items and should be renewed periodically. But renewing them will not stop a leak that has already got behind the tile — it only makes it harder to see. If the tile sounds hollow, resealing is the wrong response.
The valve, which is most repair calls
Behind the handle is a cartridge that mixes hot and cold and shuts the water off. Nearly every common complaint traces to it.
Dripping when off — the cartridge is no longer sealing. On our water this is usually scale on the sealing faces rather than the part wearing out.
Stiff handle — the early warning, and worth acting on. Stiff handles get forced, and forcing damages the valve body. That turns a cartridge replacement into opening the wall.
Temperature wandering, or going hot when a toilet flushes elsewhere — the pressure-balancing spool is not compensating. That is a safety feature failing, not just a comfort one, because its job is preventing scalds.
Will not get hot enough — many cartridges have an adjustable rotational limit stop, set at installation to cap the maximum temperature. It can be reset without any parts.
Cartridge replacement is a contained job when the valve body is sound. Take the old one to the counter rather than describing it; they vary enormously between manufacturers.
The cheap ones worth trying first
Weak or uneven spray. Almost always scale in the showerhead nozzles, not a pressure problem. Unscrew it, soak in white vinegar for a few hours, scrub the face, rinse. Do this before concluding anything about your plumbing — a scaled head produces exactly the same complaint as low supply pressure.
Slow draining. Hair bound together with soap scum, which is what soap becomes in hard water. Mechanical removal works; chemical cleaners do not touch a hair mat and damage older pipe.
Gurgling when a toilet flushes. Not a shower problem at all — that is a main line symptom, and the shower is simply where it is audible.
Discoloured grout and a film on the glass. The film is calcium rather than dirt, which is why it comes back so fast. It is a hard water symptom, not a cleaning failure.
If you are replacing the shower
Three things determine whether the new one lasts, and only one of them is visible afterwards.
The waterproofing. Tile and grout are not waterproof — grout is porous by nature. What keeps water out is the membrane or waterproof backer behind it, and getting that right is the entire difference between a shower that lasts twenty years and one that fails in five. It is also the part that is invisible once tiled, which is why it is where corners get cut.
The valve, chosen before the walls close. Whether it is pressure-balancing or thermostatic, and whether the body allows the cartridge to be serviced from the front, determines every future repair. A valve body that requires opening the wall to service is a decision you make once and live with.
The drain and its slope. The floor has to fall correctly to the drain, and the drain assembly has to tie into the existing waste correctly. Getting the fall wrong leaves standing water, which finds every weakness in the waterproofing.
This is also the moment to add a shutoff or an access panel if there is a sensible place for one. It costs almost nothing during the build and saves opening tile later.
Order of work, if you are coordinating trades
The rough plumbing goes in first — valve body, drain, any relocated supply. That happens with the walls open, and it has to be right because everything after it is buried.
Then waterproofing, then tile or the surround, then the finish plumbing: trim plate, handle, showerhead, drain cover.
The mistake that causes the most trouble is choosing the fixtures last. The valve body is installed during the rough-in, and it must match the trim you eventually want — so the decision has to be made before the walls close, not after the tile is up.
If a shower is being moved rather than replaced in place, the drain has to move too, and on a slab that means breaking concrete. That single fact is often the difference between a straightforward refit and a significantly larger job, and it is worth establishing early.