Complex sentence: Princess sales although strong for the 1976 model year tailled off quicker than they should primarily because of the associated quality and reliabilty issues but also by 1977 many of its competitors had a fifth door.
Split into simpler sentences: Princess sales although strong for the 1976 model year tailled off quicker than forcasted primarily because of the associated quality and reliabilty issues. However by 1977 many of its competitors had gained a fifth door.