Humidity is exactly what I was thinking.
You had posted earlier that the humidity and heat was just to much to be out shooting.
My suggestion; as was already mentioned, Use that magnifying glass and look for "shinny" spots in the mortise...Dykum Blue may be needed, but these shinny wear spots are relatively easy to remove and make all the difference in the world, even in the accuracy department.
The problem with hardball shooters like you and myself, it's extremely hard, at first, to forget everything we learned over a life time of shooting Centerfire, but we have to!
Sidelock front loaders are a whole 'nother ball-game, even our powder behaves differently.
Of course, you know all this already, it's just requires a different mindset when it happens.