Sounds like you set up a container field with an auto-entered calculation. Existing records will not update automatically when you add or change such a calculation.
Change the field type to Calculation instead of container. You can then open the calculation definition and make sure that the result type is specified as container, but I believe that assignment will be made automatically when you make the field type change.
If, for some reason, you want to keep the field as an auto-entered calculation, see this thread: Updating values in auto-enter calc fields without using Replace Field Contents