Sounds like a breechplug issue to me personally, I don't think the powder is getting enough primer flame to properly ignite, I bet your breechplug is fouled from the 777 to the point of constricting primer flame, plus, your flash channel and flash might be just big enough to ignite BH209 when squeaky clean, but not enough when fouled, it might need to be modified or replaced with an aftermarket replacement if available, just my $.02.
