I've always went with 2-1/2:12 is the lowest pitch that can be shingled. Anything lower would need a hot tar or heat seam rolled roofing. The pitch is too low. But if it was good for several years, I would suspect a flashing or something else is the issue. The transition from the 6:12 to the 2:12 should have been flashed with something. You could get a tube of tar and go up looking for areas that are suspect and dab on some tar. You might get lucky.