I′ve got the same problem: my left pec has always been slightly bigger and, thus, stronger than my right pec. As a result, my right lat, shoulder and triceps are a bit stronger than my left it seems.
Interestingly, Ellington Darden wrote about lack of symmetry between bodyparts (the arms in this case) in his book ′The New High Intensity Training (p. 49). And I quote:
"If you watch closely at a major bodybuilding championship, you′ll notice something interesting: Almost every right-handed bodybuilder will have a left arm that′s larger than his right. This is because the left arm of a right-handed person must work harder to perform its share of an equally divided workload. The left arm doesn′t work more, and it doesn′t work differently. It simply works with greater intensity. And it responds by growing larger than the right arm.
Let′s say that you′re right-handed. You right arm, obviously, is better coordinated than your left. So your balance and muscular control are less efficient in your left arm. Therefore, your left arm works harder, and its response to this increased intensity is to grow larger.
In fact, in tests of strenght that don′t involve balance or muscular coordination, the left arm is almost always stronger than the right, as well as larger.
When this interesting fact is brought to the attention of most bodybuilders, their response usually is "Well, in that case, I′ll do an extra set of curls and extensions with my right arm. Then it′ll grow larger, too."
Which, of course, brings us back to the point of this book: To achieve growth, you need HARDER exercise, not more exercise."
What helped me mostly is when I switched from Flat Bench Barbell Presses to Parallel Bar Dips (make sure the bars are equally placed and symmetrical!). I suggest you do the same: concentrate on Dips for a while and see if that helps. Dumbbells seem to aggravate the problem, so don′t use them unless absolutely necessary (for Dumbbell Flyes and Dumbbell Lateral Raises, only). Use (Olympic) Barbells instead, or, preferably, a good Smith Machine or other good device.
Last but not least: use SUPERSTRICT form in ALL of your exercises (the 4-2-4 cadence is a helping guideline, but slightly slower is O.K.).
Hope this will help you. Good luck!