He COULD be the cause that you cannot gain strength or mass.
1)With a spotter/partner, some people tend to overload the bar too much and do partial forced reps.
2)Too heavy a weight which REQUIRES assitance may tear your muscle fibres instead.
3)Chit-chatting for prolonged periods
4)Your training partner may be an asshole who likes to fool around whilst you workout.
5)With reference to number 1.By the time you spot(or should i say help) your training partner.It would affect the weights you can lift and so on.
Frankly i have helped this guy with his 100kg bench(says he can do it himself without help)In the end we played catching.I wanted to rerack it for him when i was sure he would not rep out more than 2.He kept lowering the bar until the 6th rep.WOA it gives my forearms and traps a greater pump or tightness than deadlifts