I'll try to explain what I mean a little less technically; doing more "pulling" isn't necessarily what I mean as some of the exercises I am describing could be seen as "pushing" they are just in a different plane from any of the pushes you are currently doing.
First I'll try to briefly describe the joint actions I am talking about:
1A) Shoulder Flexion- Imagine you are standing (sideways) along side a wall with your right shoulder against the wall. With your arm straight your slide it up along the wall until it is in a straight line with your body (overhead).
1B) Shoulder Flexion- Now imagine that you bring the arm back down the wall, but instead of stopping when it's by your side your continue behind your body until your arm is parallel to the ground (creating a 90 degree angle to your torso) behind you. Technically, anything from along side your body back is considered "hyperextension" from a kinesiology standpoint, but whatever.
2A) Internal rotation- Imagine your are now standing with your back to the wall, you raise your arms out to the sides (a motion called shoulder Abduction) until they are parallel to the ground. Next, bend your elbows until they are at 90 degrees. Finally rotate at your shoulder joint trying to bring your palms to the wall while attempting to keep your upper arm (shoulder to elbow) parallel to the floor and against the wall.
2B) External Rotation- Now imagine that, starting in the internally rotated position, you bring your hands off the wall and attempt to rotate at your shoulders until the backs of your wrists are against the wall (with the upper arm still against the wall as well as your back).
3A) Shoulder Horizontal Adduction- Imagine that you are again standing with your back to the wall and arms out to sides with the arms straight. Next, imagine that you bring your arms across your body (like you were sliding them across a table that is shoulder height) allow them to cross. Ultimately, actively this motion will generally be limited by things like muscularity, but passively (you pull the arm across with your other arm or lay on your arm) you should be able to get the arm to touch the opposite side shoulder (with the elbow locked straight).
3B) Shoulder Horizontal Abduction- Next imagine that you start out in your fully Horizontally Adducted position, but now move away from the wall. You now bring your arms back out to your sides (again imagining that they are sliding along a table that is at shoulder height) but, instead of stopping where the wall would have been you continue to bring the arms behind you as far as you can. Technically the line between Horizontal Abduction and Extension become somewhat blurry once the arms get behind the body, but, with some assistance (holding onto a bar or walking the hands along the floor while seated) you should be able to bring the pinkies together to touch behind your back with the arms at 90 degrees to the body.
Each of these movement groupings are "antagonistic" meaning that the muscles that perform one, are opposed to/stretched by the muscles that perform the other.
Boxing is very heavy in Shoulder Flexion (but only a partial ROM), Horizontal Adduction, and Internal Rotation and although you do technically have to return your arms back to a guard position (meaning you are doing the opposite motions), you are rarely if ever doing so under load/resistance. Your resistance training program is also heavily focused on the same motions as Boxing and lacking in the antagonistic motions (and the few that you are performing don't take you anywhere close to full ROM in those motions).
I'll try to post some examples of what I mean exercise wise when I get a chance.