Well, I'll start...First off welcome, these guys will all chime in and give you plenty of options...
A couple of things...First, did you put grease on the threads of the breach plug??? Second, 777 is known to leave a crud ring on some muzzleloaders with some primers...Third, as mentioned, make sure you remove all oils, grease, lubricants from the barrel before shooting...
As far as your dad's gun loading easier, you can adjust that by changing sabots...Some guns have a tighter bore than others...Both MMP and Harvester make sabots with different thicknesses to account for bore size...
Now, my personal opinion...I don't care for pellets, 777 or PowerBelts...I still shoot real black powder as I also shoot flintlocks so I have a good supply...If I didn't shoot black, I'd shoot loose Pyrodex R-S...Each shot with a muzzleloader is a hand load and I prefer to be able to experiment with different powder charges to find the optimum charge for accuracy...
Also...I don't care for PowerBelts, there are more aerodynamic projectiles out there and since velocity with a muzzleloader is pretty low I prefer to use them...As you gain experience you will see the advantage of buying bulk pistol bullets and sabots to match what you plan to hunt...
Good luck and enjoy...