If the antenna motor runs and you have even minimal movement of the antenna, it would seem to indicate that the antenna drive mechanism is still good. It's likely that the clutch is slipping.  To adjust the clutch:

• Take the antenna out of the car.
• Take off the rounded (semi-spherical) cover. It will pop off with gentle prying along the edge.
• There's a clutch mechanism that consists of a rotating plate and two ball bearings. The clutch assembly is locked to the drive shaft with a pin. The clutch pressure is adjusted with a nut on the drive shaft. Make sure that you clean off all the old lube (that tends to be everywhere), then loosen the lock nut, and tighten the adjusting nut just a little (1/4 turn at a time). Don't tighten it too much, as it needs to slip when the antenna is all the way up or down.
• Repeat as required until the unit goes up and down.