I don't think there is a trick to building big pecs.I was in the same boat as you, big arms and shoulders, small chest.Like you i did flys, dumbells, barbells and various angles on the bench.The truth is the best excersise for building the chest is the flat barbell bench press, there's no way around it.So why isn't it working for you?same reason it wasn't working for me...tecnique.
I lifted too heavy raised my shoulders and took the focus off my chest.Heres what you do,forget about the weight your using.Keep your rep range around 8-10 perfect reps.Work on your tecnique - squeeze the shoulder blades in, grip the bar hard,lower under control and gently tip your lower pecs,raise smoothly and contract your chest hard.Focus on the chest stetching and contracting.In between sets stretch and flex hard.Every week add one rep or a small amount of weight and never sacrifice your tecnique.Train your chest first in your workout followed by tri's.
Now all i do is two or three work sets on flat bench and for the first time in a long time, my chest is growing.Forget about all the shaping movements until you've got something substantial to shape.
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"I'd eat a kilo of shit if i thought it would give me bigger muscles"-Arnold