The Natural

ok, well definition will come from having a lower body fat, which will be more to do with your cardio volume and diet.
if your shoulders are lagging it might be due to the fact that you are training them last in the week, whereas you might have more energy on a monday....also training tri's the day before shoulders will only hinder the amount you can lift(on the pressing movement).
maybe try doing shoulders on a monday and going with a new routine, what do you do now?

cheers.