It could be that the EOS (End of Stroke) switch is opening too soon as the flipper travels across its arc. The switch should just open as the flipper reaches the end of its stroke and not much sooner. The more the EOS switch is open during the flipper's travel the less the high powered coil is operating on its own. If you've trapped the ball the EOS switch is open. If you then let the flipper relax but the EOS switch doesn't close right away you'll only get a weak kick because the low power coil is still engaged.
A better explanation and demonstration are in the video at https://www.funwithpinball.com/exhibits/small-boards#Flipper