Lagging Body? Try some of Rich's Tips
https://www.youtube.com/watch?v=JrA5zL9u5NE
I personally can say that my arms were always shit.....year and half later they're 1.5 inches bigger and leaner. I don't care if you hate the guy, some of what he says works.
*He does believe in overtraining if you are natural, he clarifies this in another video. So if you are not on cycle some of these methods may not be suitable for you, especially intense training three times a week. But hell, why not intense training once a week and two days of easy training like 5 sets. Push yourself. F*ck average.
https://www.youtube.com/watch?v=KlUCEB3-64g