A few points:
Keep the elbows in.
Drive with the back leg/leg that is the same as the hand uppercutting.
Bring the uppercut in/up diagonally.
The further the hand is from YOUR body, the LESS power.
Keep the hand loose BEFORE the punch starts and tighten as the punch approaches the target.
Drive INTO the body ( aim for the spine)
Torque the waist INTO the direction of the uppercut.
The further away the ELBOW from YOUR body, the LESS power.