People are going to chastise me for saying this, but I use WD-40. Most of the time it works, but sometimes it does not. Then and only then, I take the unit apart.
For Gottlieb Decagon units, I spray them in the center where the reel is attached and all joints. I hold in the coil then I spin the reel several times to loosen it up. Apply more WD as needed. Works almost every time. If the unit does not do a clean 9th position, on the Gottliebs I adjust the switches slightly until the 9th position gives a good clean snap.