Diet and not training is what plays the largest role in bulking as long as you have proper rest. Training shoulders the day after back isnt always good, and neither is training tri's after shoulders(cause tri's are secondary muscles in a lot of the shoulder exercises). What I would reccomend doing is putting shoulders and arms on the same day and taking a third off day to give you enough recovery. Up your cals(there are websites and formulas to calculate about how many cals you burn a day, so you would need to take in more than that to gain, which most people dont do) but keep them clean and you should gain some good weight.
*when i said to group shoulders and arms, i mean to put them both on saturday so you dont have two off days in a row, and so your biceps can recover a bit after the back workout.