Also look at the bottom of the bolt head to be sure it does not need to be filed flat.
The tank seals are generally neoprene and seal easily to the porcelain. Any machine or mold marks on the bottom of the bolt head can be the source of leaks.
The latest one I used included 4 neoprene washers one pair inside of tank sealed by by the bolt head with use of a brass washer and nut tightened against the tank before setting tank on bowl. Once assembled like that the tank tank with "studs" then dropped onto the bowl and the second set of neoprene washers installed and held by another brass washer and wing nut. The instructions it said the middle washer and net were not required, thus I left them out and never had tank seal leak (3 toilets done this way
Make sure to clean porcelain and insure other fittings are sealed tight as well It may not be the bolts. I do not use any sealants or grease unless recommended by manufacturer.