The leaf or microswitch actuator can be mal-adjusted with too much resistance. The rod going through the field can be installed backward allowing the loop to drag on the playfield hole. The bracket can be bent or twisted creating drag. Wear on the wire and bracket holes can cause drag. Apply 1/8 drop of oil to metal contact points.
[Edit] also the wires coming out of the side of the spinner can be bent up or down. Even a *slight* bend here can cause a huge reduction in momentum.