The Pectoralis Minor can be tough to get at with the roller. Here is a video of me using the foam roller on it. The roller lies perpendicular to my arm, and I try lift my arm up from the ground while I role. This activates the muscles in the middle of the back, which effectively help stretch/relax the Pecoralis Minor.
This helps prepare my shoulder for any pressing or overhead exercises. Try it for several minutes during your warm-up next time.