Kodak is known for consistency. Did you shoot both with the same camera, lens, lighting conditions? Shutter problem, perhaps? Light meter battery going bad? Something along those lines...?
I would probably roll up a short roll with 10 frames on it, and then put a factory roll in and burn off a few frames on that too, using the same body, same lens, same meter, etc... Just to make sure, unless that's what you already did. Then develop them together again.
If the same problem persists, I would say you have defective film. Perhaps storage issue of where you bought it could be a factor too.