Why at times can the OU not tell me if something is made from kitnityot?
All products are made from ingredients, and ingredients are often made from sub-units. For example, a candy generally contains a flavor, and the flavor may be made from as many as 30 ingredients. Some of the ingredients in the flavor are made from other ingredients, as well.
At best, we can suggest the potential existance of Kitniyot or Chametz in a product that we do not certify for Passover.
If you are Sephardi, consume Kitniyot on Pesach, and therefore need a list of such products, we recommend that you discuss such questions with a Sephardi Rabbi.