What kind of spindle are you using, 24V or a router? What's your belt tension set at, and how do you check it?
Problems causing lost steps are typically from loose or missing pulley screws, V-wheels that are too tight/loose, belts that are too tight/loose, or pots (as you've checked). I would check them all, even if you find one problem, since there's typically more than one small problem leading up to big problems like that.
You should be able to turn the V-wheels with a bit of effort, so if you can't spin them then they're too tight.