@andy52 touched on it,, Pressure.
Think about what we're doing with ignition,, we're turning powder into hot expanding gasses in a confined space.
Those gasses want out, most of it pushes an obstruction forward (the patched ball/projectile),, but it seeks all exists., some of the gas comes out of the nipple or the flash channel.
We call it gas blow-back . The best scenario is to keep that blow-back consistent with each shot. The small orifice of a nipple, along with the hammer spring and cup covering the nipple, limit's the amount of gas escaping to the rear but some does come out.
New nipples eventually succumb to "gas cutting", that's the process of the hole in the nipple getting larger by damage from the escaping gasses, depending on care that can happen as early as 200 shot's but normally around 4-500 shot's.
When gas cutting get's bad enough, the escaping gasses can blow the hammer back far enough to catch the half-cock notch.
We call that "hammer blow-back"
Basically, the whole mess leads to inconsistent gas release to the rear, lending inconsistent pressures left for pushing the projectile.
Get it?
Your target in the other thread show's "minute of Deer" accuracy well enough for most folks, you'll be fine, I wouldn't try changing anything this close to season.
@longcruise is right, your 90grns is kinda hot for the 50, and the .010 patch a little thin.
But you can work on all that for next season.
Good luck with the Saturday season,, make some meat.
p.s. When I set-up for the hunt and want reliable ignition, I load the charge and projectile, then trickle a tiny bit of powder under the nipple in the flash channel
