With a sidelock, the gun is designed so that the cap lights up the powder that flows into the bolster, underneath the nipple. That is why you tap it on the side after you load the loose powder, before you seat the bullet. With pellets, nothing can flow into the bolster, as your propellant is one solid mass.
Further, Pyrodex pellets have a BP coating on one end to facilitate ignition. In an inline, that end is towards the primer. With a sidelock, your cap is to the side, and the BP coating in at the bottom of your barrel, away from the cap.
Pellets were never intended to be used in sidelocks.
Stick with loose powder. I use T7 FFFG, and Pyrodex RS in my .45 TC Hawken, with good results.