Lagging right pec due to shoulder pain

[Yello_13]

I was just wondering if i could get some tips on building a lagging pec. My right upper pec has very little muscle compared to my left upper pec. I get a sharp pain in my shoulder every once and awhile when i'm lifting for chest exercises. When doing any kind of chest press i find my right arm always struggling while my left still could do a couple more reps.