I always give the tiles a quick wipe with rubbing alcohol- that makes sure you don't have any gook on them that might leave an un-even stamp. The only other thing I can think of is that you might be over inking? It just takes a light tap tap to get enough Stazon to make your image. If you tap too hard you get a lot more ink and it becomes harder to keep your stamp steady when you press. Like mentioned above- those white tiles get really slippery! The extra ink also tends to "push out" when you press- remember that the tile won't absorb any ink like your CS does. I've only had uneven color when I forgot to clean the tiles before hand... make sure you are tapping the color on with your ink pad and not sliding the pad across the surface of the stamp.
You can always apply a thin layer of color to the tile before hand- like a thin thin wash. That sometimes help with the stamp not sliding as much. Just dip a paper towel in the corner of your stazon pad or some of the adirondack alcohol ink and rub all over the tile. That might help you out
Good luck!