I was thinking the same thing as leonf. It sounds a bit like when oil has seeped out on the belt. It needs to be whiped off together with the groove in the handweel and motor pulley and it should grip as new again. Similar can happen when a belt needs to be repaced, but lucikly it's visible and obvious when it's time.
If it's a model with a potted motor it could very well be sticky old grease, often you need to take off the hand wheel to check behind there. The worm gear and tooth in the hand wheel are often much more sticky than the motor axle.
Sticky parts in general are yet another common cause for this problem, and it's never a bad idea to go over all the oil points a few times and run the machine a bit if it hasnt been used it a while. It can take time to flush out grime and dirt from the inners of joints and gears, all it takes it a bit of persistance and daily oiling for a while.