for each exercise you warm up ?? i only do a few warm-up sets on the very first exerscise (not back day b/c i start of with wide grip pul ups). after that i start off about 75 - 80 % of what i'm goin to do that day on each new exercise. too many warm up sets can just burn your energy that you could be using on heavier weight.

if you're doin 80 - 90 % of your max and you can do 8 - 10 reps, i would say your weight is too light.

also, do you not incorporate shoulders anywhere in your workout ??? shrugs, press, lateral & front raises, etc. ??