It might help a bit if you described how you're doing them, especially whether you're holding the dumbbells so that they face each other, or face outward throughout the lowering and lifting. Note that when you perform flyes with the palms facing each other, your biceps are recruited somewhat more than when your palms are facing outward. In fact, when I do flyes with the palms facing outward I often have to cut reps a bit or go with a slightly lower dumbbell weight load because the biceps aren't the helpers much!
Also, how low are you lowering the dumbbells at the bottom? One thing to pay attention to is not "over-lowering", so that the elbows and the weight load go lower than whatever platform (weight bench, step bench) you're laying on. That will recruit and often overstress the anterior deltoids, which are assisters in the flye to begin with.
Let us know how you're performing them, and if you're not doing flyes with a palms-forward position, give that a try - carefully!
Also try varying the position of the trunk, by doing flyes on a flat bench, and then on an incline bench with your head at the higher end.
Hope this helps -
A-jock
A