Locate the crack
Is it along an inside corner, where wall meets floor, or between field tiles? Corners and changes of plane experience movement; the right joint detail depends on the installation. Schluter notes that inside corners are subject to movement and may use a flexible joint or movement profile.
Look beyond the grout line
Check for soft walls, staining in adjacent rooms, swelling trim, a persistent musty smell, or loose tile. Document where the water appears after use. Those clues help distinguish surface maintenance from a leak or waterproofing failure.
Avoid sealing a hidden leak in place
New grout or caulk may improve a sound joint, but it does not rebuild damaged waterproofing behind the tile. If moisture is escaping, limit shower use until the source is identified. Do not assume a fresh surface bead will stop water traveling through a failed assembly.
Plan the right repair
A stable shower with an isolated failed joint may need cleaning and a joint repair compatible with the existing system. Repeated cracking, loose tile, or wet material may require opening the area and repairing the assembly. Photos of the entire shower and the damaged joint are helpful.
Technical reference: Schluter: movement at shower inside corners ↗