Are generic drugs inferior?

A friend who is on depression medication has tried several kinds and has found one that works for her. Her insurance has decided they will only cover a generic version of said drug. She has tried it in the past and swears it did not work for her the way the brand name one does. She has said it is not just the side-effects of the generics but the way it does so much less for the depression. Her doctor concurs with her and between the two they are fighting with the insurance.
So, is she correct? Would not the main ingredient of a generic be the same as the original? If so, why would it not work as well? Is it the buffers? Can a generic use different buffers and still be the same drug? Could it be that the actual drug is enhanced by the buffers and if so, are they still buffers?