Foodie Sue, (why is that your screen name, btw?)
I think everyone has their own particular strong and weak areas. I've always felt stronger in the biceps, but still struggle with the upper back area. Over the past couple of years I've managed to meet Cathe's weight in all of her workouts, and then surpass her in many of them, with the exception of some of her upper back work. I have yet to even get to her weight for that work in MIS and Body Max. In newer workouts she's gotten lighter during those, and I do okay, but it's still my weak area. I really dislike bent over rows for this reason: PUB, CW, and the SH sets are particularly difficult for me, and I can't forsee going any heavier than Cathe any time soon.
Do you have the Slow & Heavy Series? I found that series to be the most effective for helping me make actual strength gains.
Sandra