I shot my Investarm .54 Hawken again today at the Boyertown fun shoot and it turns out that I wasn't having cap problems at all. Rather, it was that the fly in my lock was not working properly and on many of the drops wasn't allowing the hammer to fall past the half cock notch if the trigger was set before firing. I pulled the lock, made sure the fly was moving freely and lubed it with a couple of drops of Ballistol. After I did that I had no more ignition problems.
Tomorrow I'm going to remove the lock and blast it out with brake cleaner, then relube with Shooters Choice FP-10. Unlike Ballistol, FP-10 doesn't gum up and is good down to -40F, which is 50 degrees colder than I ever plan to be out shooting or hunting.
I plan to do that with my other sidelocks as well.
Note:
Sidelocks meant for use on muzzleloading rifles or pistols with set triggers have a part called a "fly." It is a small, pivoting piece that when working correctly, prevents the half cock notch from catching on the sear. A fly is not needed on a lock used with a gun that doesn't have a set trigger, because of how the set trigger kicks the sear when the gun is fired.