You may have stripped the drive gears - I did that myself with the slicer attachment on a Model 11! Take off the front cover plate and check the gears - you'll be able to see right away if they're stripped or not meshing properly. If they're stripped, they can be replaced easily enough. Depending on which model you have, you could probably find a parts mixer at a thrift store or on eBay. If not, I have a bunch of Model 10 and Model 10A parts mixers that I would be happy to 'part out' if this would help.