Yeah! I had this problem with my lower back. I did rowing at the bench with dumbells, not healthy for your back bone, but still...
My laft rower could row 16 reps with a 16 kilo dumbell, my right could barely do 12 with a 10 kilo dumbell, they looked the same, so I tought that it was my stand that's not good (caz when rowing with free weight you change your stand for each side), but the instructor said I do it perfect. All I had to do was to stick with the 10's and slowly increase while making sure the right rower is catching up. Now I am completely even, rowing 22 kilos at 16 times like nothing. Gonna start rowing with the machine soon...
I have an idea why can it occur:
-Breathing problems (breathe well before, during and after workout).
-Blood Supply (check your blood vessels, your shirt, your fat levels, etc...)
-Warm Up
-Stretching
-Position
-Speed of performance (make sure both sides work at same speed)
-Nutrition
BlahBlahBlah... Basically, I am drunk and tired right now... Sorry for the lame post...