Right upper body is weaker the left...why?

When I do weights at the gym, it seems that all of the muscles on my right side can only do 70% of what my left side can do.
This includes traps, pecs, deltoids etc.

The only thing that is apparent is that my scalenes seem to be very tight, I have reduced left cervical rotation and it feels that there is a bunch of tightness near the insertion of my right scalenes.

I have no pain or pins or needles but slight reduced sensation on my right arm.

When I stretch my median nerve ie.external rotation of arm, extend arm and stretch neck away from stretched arm, there is a more apparent nervous stretch on my right side which leads me to believe the nerves are restricted by some structures near my neck.

What should I do to rectify this problem- daily stretches? Manual therapy?

Thanks in advance for any suggestions.