All of these comments are helpful.
I followed OldNo7's guide on deburring and cleaning the flash channel from Jan of this year. On top of that, I lightly deburred the front edges and interior of the powder drum with 0000 steel wool and cutting oil on a shaped bronze cleaning brush on the end of a long cleaning rod with a bore guide chucked into a power drill at low speed.
I then followed the instructions on tipping and tapping the barrel to help gravity when I put the powder charge in the barrel, and removing the nipple to see if there is powder in the channel. When I ran the experiment with the 3F/FFFG (black powder), I saw it, but not when I tried the 2F/FFG (T7). I don't have any 3F T7 on hand to verify the result with that powder.
I expect that making sure there were no obstructions in the powder drum and flash channel, and using the smaller kernels of 3F vs 2F will solve the problem. I'll know for sure when I get to the range after the rains clear.