I've used nothing BUT magnum LP's (CCI & Wolf) in my BP 44-40's for 25,000+ rounds so far.... using between 28 - 35 gr FF and/or FFF (Kik, Schuetzen, Goex, Elephant and Diamondback), big-lube 44 bullet (sized .428), homemade lube, many combinations of 50/50 lube using beeswax + any/all of the following: olive oil, crisco, deer tallow, mutton tallow. No fouling issues with over 100 shots fired at over 100 yds w/o cleaning in 1866 rifle. Rapid fire, from X-sticks, the rifle held about an 8" group for 100 shots fired at 110 yds using the barrel sights. I now am shooting my 66 carbine with equal result..however, the 19" barrel is an advantage for less fouling. In fact after a 60 round match, the end of the muzzle is absolutely wet & oozing with lube.
IMO, The lube formula combined with your weather (temp/humidity) is likely the issue. Persoanlly, I size/lube my bullets and load them within a day or two at most. I've observed my own stored bullets (sized & lubed) for my Sharps rifle experience diminished lube performance after sitting for a few months before loading them. I suspect moisture in the lube ingredients dies out sitting exposed to the air, while my loaded ammunition has performed well after sitting for 2-3 years.
Slim