You have already scrubbed it. Bleach first, then a paste of something, then a stiff brush and both knees on a wet floor. The tile came up fine. The lines between the tile stayed grey, and that grey is the reason a clean bathroom still reads as dirty.

Grout is porous. The grey is not a film sitting on top of it. It is soap residue and dirt worked down into the body of the material and held there. A brush works on the surface. The color you want gone is under it. That is why the third pass looks the same as the first.

This lays a new white surface straight onto the line, so the old color stops being the color you see. That is painting, not cleaning, and it behaves like painting. On a line that is sound and dry it reads as new grout.

The prep is the job and the pen only finishes it. Onto a line that is damp, greasy, or still holding cleaner from your last attempt, it goes on badly and lifts early. Your scrubbing was preparation, not the fix. Get the line dry and clear of cleaner, then color it.

Four stars from 32,755 ratings, and I am not dressing that up. That is a middling score and you should weigh it. My reading is that a surface treatment gets judged on the surface it went onto, so a share of those low scores happened before the pen touched the line. That is judgement, not evidence.

Here is who it is wrong for. If your grout is cracked, crumbling, or missing in places, this is not the fix. It recolors a surface, it does not replace grout, and a fresh white line over a failing one only makes the failure easier to see. Repair first, color second.

And a shower floor will want doing again well before a wall does, because feet and standing water live there. On grout that is intact and only tired, it does the one job it claims. The line goes white, and the room stops looking dirty when it is not.