There are two things that can make it difficult to keep your tiles clean:
The easiest way to test for detergent build-up like this is to pour a couple of tablespoons of water on a small area of the floor and scrub with a green Jiffy pad. If you can get suds to appear, the floor is soapy.
Both problems have the same simple solution. All you have to do is clean the floor properly with Heavy Duty Tile and Grout Cleaner, which will remove the dirt and any detergent build-up. It is vital to follow the directions and rinse thoroughly after use: otherwise you are just leaving the same problem there again.
If a fine grout haze is still present after cleaning and rinsing thoroughly, we recommend using Phosphoric Acid Cleaner with a Poultice to remove it. As with any specialised chemical cleaner, follow the directions closely.
Keeping your tiles clean is really easy, and actually requires very little work. Take a look at Routine Cleaning page in our How To Tile DIY Guide.