I have counseled folks on the fitment of their projectile to make sure that they sufficiently seat securelyin the bore. One guy I worked with was shooting a CVA 50cal sidelock and somehow he got alhold of .44 cal roundball. He shot about a pound of poweder and rarely hit the target.
Got ahold of me through a mutual friend and I was able to quickly deduce that this load was too loose in the bore.

Also a good safety rule of thumb is:
"Anytime your ML is jarred or dropped, re-verify that your projectile is properly seated in the bore on top of the powder". I have had them jar loose slightly after dropping my ML(PRB's and conical's).

If shooting a powerbelt for sure verify they are seated if you jar your ML as they are a "slip-fit" conical. I think the MFG recommends this.

Tahquamenon