have you been training them, equally.
I.e. if the left gives out quick training with the right.
I believe 1 sided movements (such as 1 arms dumbbell rows), while effective, can sometimse be hard to match the same intensity that you did for each side. Sure you may have done the same reps, but was the concentrations there, did the follow the same exact angle upwards ect... Personally I stay away from them simply because I can't always get the same kind of concentration from one side to the next. This isn't true for everyone, but for me it is.
Stick with movements that work both sides at the same time, and dumbbell choose over barbell where possible since you can influence ones sides actions with the other with the bar slightly. When the weaker sides gives quit the set.