I have one of these too... Also paid very little at Goodwill. I damaged mine trying to adjust the feed dogs and took it to the shop. We don't really have any tech service for newer machines, but this guy loves to tinker with the older ones.... Any way, if yours is like mine, it is open on the bottom. If you turn it on its side you can see underneath what the knob is doing when you are changing it from high, low, down. There is a long flat lever that moves to change the position. Maybe that lever is bent (which is what was wrong with mine) so that it isn't really moving the feed dogs fully. If so, the repair person here straightened mine and it works ok now. Or maybe a connector is off so it doesn't move properly.