If some stains refuse to move, do not immediately reach for the harshest bottle under the sink. First, repeat the paste treatment on only the darkest lines and give it a little longer to work. Stubborn grout usually improves in layers, not in one dramatic swipe. It is better to clean twice gently than once so aggressively that you damage the surface.
A steam cleaner can also help, especially in showers where old soap film and moisture have been building up for months. Use the small brush attachment, move slowly along the grout, and wipe away the loosened dirt as you go. Steam is useful because it lifts grime without needing a strong smell or a heavy chemical cleaner.
For white grout that still looks yellow or grey, an oxygen-based whitening cleaner can be used carefully according to the label. Always test a hidden spot first, especially if your tiles are natural stone, coloured, textured, or old. Never mix different cleaners together, and avoid using very acidic or very harsh products on delicate surfaces. Bathroom grime is frustrating, but damaged grout is even worse. Patience protects the finish while still giving the cleaner time to do its job properly, too.