over training, under training, lack of focus on pectoral aspects, poor movements, secondary muscle involvements, form, etc etc. There can be many reasons for lagging body parts. If everything else is going fine, youre still growing and getting stronger in other areas and gaining weight, perhaps you might be better off posting your chest routine and seeking critique there.
Also as i said, form can be confining for growth. For example, If you Fly, perhaps you focus too much on touching your palms at the peak and not enough on your ROM on the distance back, The actual reason for the fly.