You said you are at the low end of Retumbo is that what you mean by the starting load ? the reason I ask is because I load Retumbo at 85.5 grains in my .300 Rum and if I remeber correctly thats my starting load and something like 91 grains was the upper end of the load data that I started with. I can say this for sure tho , I started reading off the net about how everyone was pushing retumbo in the 91 to 94 grains . When I tried to load my rifle with New Nosler brass at these charges I would have ejecter marks and sometimes stiff bolt lift. I was told when I asked a similar question as you have that this would happen as the nosler custom brass is softer than the Remington brass . In short I started using the remington brass again and havent had a problem this far but i am not loading on the HOT side either.
Just an ideah to think on