Hi OSTexo, well at first I thought there might be a micro sized edge in there that was not getting selected by just clicking on things.

But doing a window select around the hole would only come up with 2 edges being selected, so I ruled that out.

Then just one of the ways I tend to inspect a problem model more closely is to delete chunks of it and see if there are any weird little bits left over or things like faces doubled up over top of themselves or stuff like that.

- Michael