The chain has a master link in it. You can pop the side off and split the chain. This is what you have to do to adjust the head anyway, which, by the way, I had to do with my 16". I thought it would come adjusted but that was way wrong. With the chain off you could try each column separately. Read the manual for setting the head. I don't have the manual handy but it involved making a hardwood jig and sliding it under each corner of the cutting head to see if they were the same distance from the table. If none of the posts have a bind then I would think it must be the chain/sprocket/tensioner assembly.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.