Why do my princess house pavilion appear to be "stained"?

I have a lot of the princess house pavilion line, and a few pieces seem to be stained....it looks as if there is a "water mark" on my large pasta bowl, and a few of my dinner plates. Even my small colored appetizer dishes have a few little marks on them. Is this normal? I really like the products, but I'm not sure if I want to keep buying them if they are just going to look stained....I understand that each item is unique, but I don't want a dirty looking piece! Why is this happening to my dishes? And would you return them or deal with it?