Your muscle attachment points for the tricep and/or fiber ratio might not be the greatest.
I would guess based off your photo and what you said about your cgbp that your tricep strength is lacking in comparison to your chest and shoulder strength.
I would personally focus on fairly heavy, high rep cgbp around 12-15 reps and then as other's have suggested cable pushdowns heavy in the 6-10 range.
I prefer to keep it KISS in lifting and if the benching and pressing isn't enough I still think you are better off using the big boy tricep building exercises rather than kickbacks or skullcrushers.