[zuch]

Usually the regulator breaks. A pin that is staked to the regulator and holds the spring that helps the window go up, falls off. You can replace it with a carriage bolt from your favorite hardware store. It may require a little grinding of the head as there isn't much space at the back of the regulator. Take the door card back off and have a look around. One of my window motors was very slow. Turned out to be 40 year old grease. A quick cleaning of the transmission and some new grease fixed it right up. Feel free to send pictures. I just went through this and can probably help.