Few things come to mind. First of all, you need to access what kind of presser you are. Do you press primarily with your shoulders or your triceps? This can have a big impact on your chest development with regards to how much pec is actually being activated when you press. Do some research on proper pressing technique (tnation.com is a great resource). Try incorporating some isometric exercises before your pressing to target and activate your pecs muscles...I like to use light weight cable flyes from the bottom pulley. While pressing is important, try doing some dumbbell pullovers and/or dips as well.
Also, and I hate to say it, but you may not have the genetics for a bowling ball chest like Arnold had. Hard work and a proper workout will help but at some point we are all at the mercy of our DNA.
Good luck!