Erwan
45 Cal.
I also observed this detail...What I do know from personal experience - if you hard pack the powder when you load - it will ignite a lot slower.
It may be from the inability to spread the fire or it may be like a piece of hardwood - you know how hard it is to get that stuff to burn.
I can't tell anyone WHY it happens - but I can tell them WHAT does happen.
Being a simple guy with simple ideas, I always thought that the oxygen provided by nitrate (KN0³) was useful and necessary for combustion but did not provide enough oxygen, on the other hand too much KNO³ slows down ignition, and that it passed poorly between the overly packed powder grains and the little natural oxygen in the bulk powder that should be between the normally packed grains could no longer find its way...
Since then, I don't pack the powder too much: just the ramrod bouncing with a relatively clear sound two or three times and no more...
What made me think of this is that it is even more sensitive with flints than with percussion, and therefore the need to have a free and clean hole...
Food for thought....