Is there a rubber sleeve there? Toto supplies special fasteners with the single piece toilets and soft-close seats. The fitting slides into the hole, you attach the seat, then thread the fastener into it. There's a captive nut in the fitting. As you tighten it, the nut slides up, expanding the fastener, and holding things in place. If you have these...you may just need to press down some while you tighten the screw. It can take a bunch of turns before it starts to tighten up.