grab some light db's and lay down on the floor ...
now figure out where your elbows are pointed and see
if sliding them closer to your body or further from your
body isolates the pecs ...
also then experiment with putting your hands further out
or closer to your body ...
eventually you will find the right position for your hands and
arms relative to your body that isolates your pecs and uses
less shoulders and triceps.
its better on the floor because your elbows don't drop out of
sight and you can 'map' the best position better ...
do these with progressively heavier weights and then
try on a bench with lighter weights again emphasizing
positioning for isolation rather than lifting for max weight.
the weights will come up fast as your chest gets more involved
and the strain comes off your shoulders and triceps ...
Just remember that no one can tell you the 'right' position for
you ... its up to you to find it and you will know when you do!