Personally it has been my arms and shoulders. Luckily I don't even have to train my calves and they stay a good inch bigger than my arms (striated) and I suppose I have good chest genetics because I don't think I've seen to many naturals at my body weight (5'10 180lbs) with my chest development and I can't use a lot of weight with my chest pressing exercises. I do have a good mind/muscle connection going on there though, I feel every rep. My quads/glutes have also been very good responders over the years. My back is nothing special looks wise (I can't see the damned thing really) but on good days I can do reps of 3-5 with a 100-110lb dumbbell when I do pull ups.
And it is what everyone here is saying....it's all genetics for what will be your hard responders. Don't make excuses either way, just freaking push some weight.
