The reason the powder didn't burn was that the sabot was too loose and there was not enough pressure on the powder charge for it to ignite.
That is the reason I said that if it had been black or a sub it would have fired.
The rifle has a 24 in barrel added to 15 ft thats a long ways for the bullet to have gone with just a primer.If I hadn't seen it I wouldn't have believed it.
Allways mark my ram rods when loading if its not at the mark I check to see whats wrong.
I have been shooting over 50 years and I did load twice in a row last year without putting the powder in.When you do that and you have just a sabot and bullet in the barrel its no problem taking out the breech plug and pushing the bullet out but if for some reason you have to pull one with the powder now that is a mess to clean all the powder out of the chamber.The slightest distraction can get you problems with a muzzle loader.