It sounds like perhaps wax or something has gotten on it to dull its luster. There are strippers and cleaners that you can purchase at home improvement stores made for travertine especially. They also have some superior hard floor cleaner machines that you can purchase. I obtained a Hoover carpet and hard floor cleaner a couple of years ago and found it to be excellent. I had marks that I scrubbed for years and couldn't remove. The first time I used the machine they were gone.