do you know if the tile/grout was sealed when it was new or since? What have you been using to clean the floor? It's possible if it was sealed long ago that the sealer needs to be removed and redone. Or, if you've been using products like Bona or the like that you have a build up of product on them.

This site talks about removing old sealer. If you've been using some kind of cleaning product that could have built up, research how to remove it, probably using mild detergent and water, rinsing well, with lots of elbow grease, maybe microfiber cloths to scrub residues off. Start in a more inconspicuous corner to perfect your technic and products.

https://www.daltile.com/how-to/how-to-care-for-and-maintain-tile/how-to-care-for-and-maintain-porcelain-tile

I steam mine and they come out perfect.