I think most people will say, don't worry about it too much, just keep trying to up the weights you can lift.
A lot of people who have been lifting for just a little while tend to notice small problems such as weak upper pecs, small biceps, no good forearms. The truth is though that new lifters like you and me may have weak upper pecs and small biceps, but the rest of our muscles are tiny as well.
I am not sure what program you are doing, but listing your workout would definitely help.
Just stick to the money lifts and focus on increasing the weight
bb bench or db bench if you have shoulder problems
military press / db shoulder press
I read that bench doesn't really help certain people with their pecs and that db chest fly would do a good job.
Also, as your post is written it sounds like you have upper pecs, and then just a void of no muscle where there should be some. What kind of bodyfat levels do you have? I started fat and lost weight and the drooping skin over the bottom of my pecs took a while to go.