I don't know much about the Remington sts primer but in that cold weather I would be inclined to use a Winchester ww209 or CCI mag primers. As long as your sabot loads with a fair amount of resistance I would think BH 209 would have gone off. Blackhorn needs a tight bullet on top of the powder. I have an Accura V2 and the black crush rib sabot falls down my barrel, I can't use it. I use the SST/ML 250 also with a T/C superglide sabot, very accurate and loads great. I know it has been said that a long flame channel cools the primer gasses a lot, but I have a T/C Pro hunter and it has a fairly long flame channel and I have never had any problems using WW 209 primers. As far as your gun I would go to a hotter primer. At home on a 2 to 6 below day load your gun up the same as when you were hunting, set it outside for a couple of hours and then put in a WW 209 or CCI mag primer and try to shoot it. If it was the primer being too cold your gun should go off.

When you have been shooting your gun have you been cleaning out the flame channel? you should use a 1/8" drill bit every 3 to 6 shots using finger pressure only, if you don't your flame channel could become restricted and cause a FTF. If you haven't been cleaning it wrap black electrical tape around the shank of a 1/8" drill bit until you can grip it firmly. Then use it to clean your Breech Plug. Not inferring you haven't but just wanted to check.