Gottlieb's EM flippers are often weak because someone wound the flipper
return spring too tight. In this case the flipper is fighting the strength
of the spring. Undo the spring and wind it so that it just returns the
flipper to resting position. Note that you may have to undo one of the
screws holding the spring to the flipper bushing to move the spring to a
better position. Note also that the spring need not return the flipper to
resting position when the playfield is up, only when it is down.