Kettlebells work better than dumbbells for flyes. Because of the position of the resistance, the pecs are under load for a greater proportion of the range of motion.
With dumbbells, the muscle tension on the pecs is greatly reduced at about 60% of the range of motion. This can make the reps less effective, especially when you're using a method where you must maintain constant tension to create an occlusion effect.
Most strength coaches will categorize an athlete or client into one of three classes based on their responsiveness to training: easy gainer, hard gainer and average gainer. Let's take a look at each category.