I had the same problem with two Williams peeps aboard a Marlin 444 and my 1895 Winchester..Although not staff sights of course the problem was a PITA anyway and only rectified by changing inserts until the sight picture cleared up..My first attempt mirrored your experience to the T thought sure there was dirt,lint or some other obstruction..The only gremlin in my case were aging peepers and the wrong size hole to look through..
