For most, the solution is simply to continue working out using compound lifts with proper form – get bigger/stronger and things will even out with time. Depending on how long your progress has been stalled, you may need to re-evaluate your routine and add more total volume for the muscle(s) that you would like to be bigger.
One exception to this rule is a left/right imbalance – for instance, one arm being stronger or bigger than the other. Correcting this requires attention to proper form on exercises, to discourage leaning or twisting to favor the stronger side. Additionally, unilateral work could be done on the weaker side first, with the stronger side completing the same weight and reps to match, until balance is restored.
Otherwise, review our Muscles and Muscle Groups page if you need ideas for targeting certain areas.
We also recommend reading over this thread on r/weightroom discussing Dr. Mike Israetel’s thoughts on muscle imbalances. Here’s the preface to his extensive post:
Is your body in balance? Are all of your muscle groups in balance? Or are you walking around with hidden asymmetries, obvious to all intelligent trainers but yourself? Subverting your every attempt to get stronger, healthier, and more aesthetic?
The truth is that you’re probably fine, and here are some reasons why as well as some information to help you avoid going down fruitless roads of attempted self-improvement.