Cracked grout, loose tiles and a stain appearing on the ceiling below are usually symptoms of what is behind the tiles rather than the tiles themselves. Re-grouting a failing wall buys weeks, not years.
Substrate matters more than grout
Tiles fixed to plasterboard in a shower zone, without tanking or a suitable board, will eventually move. Once the substrate is soft, no amount of sealant holds it together.
Tanking and wet zones
Shower areas need a waterproofing system behind the tile, lapped correctly at the tray and corners. This is invisible when done and obvious when it is not.
Movement joints and sealant
Junctions between tray and wall move, and need flexible sealant rather than rigid grout. Grout in a moving joint cracks every time.
When to strip and redo
Where boards are soft, tiles are drumming or a leak has reached a ceiling below, stripping back to sound substrate and rebuilding the wall is the durable answer.