You are probably just not using your pecs enough in those movements. I'd try heavy DB presses, and keep you upper arms from going below parallel. Below parallel is mostly shoulder. Since you can move the DB's together you use a lot more pec than triceps. In comparison to BB bench that is.
I'd keep some sets of bench in there.
Shoulder blades pined together, and with your elbows out a little more you'll get more pec involvement (and a
lower weight). NOTE: This strains your shoulders more.
Your triceps should be fried at this point, and you can throw in some flies, incline or flat. Same as DB bench, keep your upper arms from going below parallel.
Hope that helps.
Fudo