I had this problem for a long time, and this is what EVERYONE that lifts struggles with so don't think you're alone. The chest is really hard to build but once it starts building it builds very well. What I suggest is that you focus on high weight, low reps. This is gonna help you build mass in that area. Start your workout with incline bench/dumbell bench to get the most pump for your upper chest, which will give you the look of a big chest even if you lack in the middle/bottom. Flies and cable flies will be great for your inner chest. I can give you a sample chest workout for building if you want, just a quicky