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.

Check the bushing and ensure that you have a correct gap. They make a tool for it but it can also be done with a plastic credit card, membership card etc. If it is binding, even slightly, you will get what you describe.

**Lyonsden above beat me to it. I think he nailed it.**

Check the spring - maybe it's tighter than necessary.

Less resistance could help as well.