Check to make sure that the flipper is moving smoothly (no binding or rubbing anywhere). Make sure you have a small gap above the flipper bushing. While you say the end of strong switched is gapped correctly, is it engaging the flipper arm too soon? That will create resistance on the stroke. Check the return spring on the flipper -- has it been shortened? You can replace it with a weaker spring. Finally, you can replace the coil with a stronger one (or take some windings off the coil).
And then there is Vid's guide to rebuilding flippers. It will have more gold than want I've posted.