I had the same problem with my tiles but I can tell you about something I tried which has transformed the grout on my bathroom and kitchen wall tiles and even used it on the stone floor tile grout with great success. If you have a wilkinsons nearby, go to their DIY section & look where the grout stuff is stocked. Hopefully you will find there a tube, 125ml size, made by Evo Stik called Grout Revive. This stuff is brilliant. You just use the sponge applicator & go all over the grouting. You will go over part of the tiles too but don't be concerned about that. After 3-4 hours after applying, you just wipe off any excess on the tiles with a damp cloth. I can't praise it enough & have recommended it to others who have been equally impressed. I personally found the grout pen that someone earlier mentioned not that good but invest about �3 something in this & I'm sure you will be more than happy!