TWENTY HOURS??!!?? Why do I have a sneaking suspicion that you didn't fully read the instruction booklet that came with the cooker. NOTHING should be cooked in any slow cooker for 20 hours. Nothing. And I'm sure the instructions mention that somewhere. Not to mention LIQUID requirements, which are most important. Not enough liquid in a crockpot/slow cooker? Even after 20 hours, potatoes will still be like rocks unless there's enough liquid.
But if, in fact, you did read the instructions, had enough liquid, & for whatever reason after TWENTY hours still had hard vegetables, you need to clean your appliance, pack it up, & return it to the store as defective.