The bolts are TRASH period!! The bolts heads strip out, as you tighten. 
So, as instructions said in ad, retighten to maintain safety off stools. You can't do that with striped bolts. Unsafe stools will be results. So you will have to drill out or use vise grips to remove and replace. Deeper hex depth was needed. Received no useful help from, your agent. They seemed to have no knowledge, about hardware or use of tools. Hard to resolve issue, when you have little or no experience, on subject.