Marble Floor Looking Dull ?

It just might be your "Neutral Cleaner"

Most "General Purpose" and "Neutral Floor Cleaners" contain an ingredient that makes the water wetter (breaks the surface tension of water). These additives are referred to as "Chelates".

According to the dictionary a "Chelate" is: 
A chemical compound that acts as water "softening" agents. "Hard" minerals (such as calcium) are attracted to and can neutralize the surfactants in a detergent. Chelates do not remove or exchange hard minerals in water. Rather, chelate molecules surround and cover (sequester) those minerals. With these minerals sequestered, the surfactants in the detergent are free to foam and clean.

Makes sense right... Not Exactly

Marble is a metamorphic rock composed of calcium carbonate, (like aragonite or coral), or carbonate of lime, (limestone or dolomite, a variety of calcite).

Your "General Purpose" or "Neutral Cleaner" can actually do more harm your floor than good. Next you need a floor cleaner specifically one that's "Chelate Free".