I tried pellets in one of my sidehammer guns years ago when they first came out. They didn't work very good for me.
I think pellets are more suited for inline rifles.
I tried Pyrodex pellets last year when I bought my first sidelock, and they worked fine for me. I did end up going to loose powder, because everyone on here said it’s better.
I don’t know if it made any difference in my success, but I also use a Mag Spark ignition with 209 primers.