I am having trouble polishing a travertine floor. I can get a good polish except for the fact that I have a lot of dull spots on the floor. Why is this and is there a way I can make the shine even across the surface?

Travertine is a marble that has a lot of holes. These holes are filed at the factory with a colored Portland cement. When you polish the marble the fillers do not take a polish. This is why you get dull spots. The only way to eliminate the dull spots is to apply a coating on the surface of the marble. I would suggest a wax or a process known as recrystallization.
TIP If you need to fill these holes try using a polyester or epoxy filler. These fillers will polish giving the travertine a uniform shine. Epoxy grout works great as a filler.