[KrazyCarl92]

If the motor stalls before the physical limit, you could:
-add a reduction to produce more torque
-make sure that the motor is applying force parallel to the kicker to the path of the kicker (or perpendicular if it works as a pendulum)

If the physical limit is the issue:
-increase spring constant
-increase distance of tension/compression
U=.5kx^2
because the motor would still be able to apply enough force to pull it back further, the spring constant/distance could be increased and therefore the spring potential would increase as well.