Porcelain tile has a smooth and shiny appearance when new. Over time, though, the tile can become dull and matt looking. If this happens right away, it is usually because grout was left to dry on the tile. If it takes a few weeks or more, it's often due to soap scum or a build-up of residue from cleaning solutions. Either way, it is possible to get that old shine back as long as you are determined to make it happen.
Try cleaning the tiles with soap and water. Add 2 litres of warm water to a bucket and mix it with 1 teaspoon of unscented liquid soap. Dip a soft brush, like an old toothbrush, into the bucket. Then use it to scrub your tile in a circular motion. If you don't have a brush available, use a piece of muslin cloth.
Keep scrubbing for 30 minutes or until all the residue is gone from the tile, whichever comes first. Wet your cotton rag and wipe the solution off the tile. If everything looks good, wipe the tile dry with a second cotton rag.
Pour the soap and water out of the bucket and refill it with 1 cup of water and 3 cups of white table vinegar. Scrub the tile again with the brush or muslin cloth. The vinegar is stronger than the dish soap and can pull off more stubborn soil.
Check the tile again to see how it looks. If it has grout residue remaining, clean it again using a solution of 4 cups of water to a half cup of muratic acid. If it still has soap scum, spray it with a soap scum remover and let it sit for 15 minutes. Either way, rinse and dry the tile afterwards.
Buff the tile with a microfibre cloth to make it shine.
If the tile doesn't have residue on it but just looks slightly dull, polishing it with a dry cloth may do the trick.
Once you've made your tiles shine, you may want to clean the grout to enhance the effect. One easy way is to use white toothpaste.
Most DIY or hardware shops sell muratic acid.
Microfibre cloths can be found at electronics stores. They will not shed lint on your tile, which will help it shine.
Muratic acid is toxic. Wear gloves and goggles if you use it. Make sure you don't leave any on the tile.