The best (and in my opinion) the only way to grow lagging muscle groups is to increase the volume you do for those groups.
For example; I have/had lagging shoulders - I tried training them twice a week and they had some significant growth. I now *stimulate* my rear delts with back days, stimulate my front delts with chest days, and then 3-4 days later I do a full shoulder routine. It helps significantly, but of course everyone's body is different and reacts differently to different training methods.
Give it a shot though, the outcome may be beneficial to you!