Try shooting from a bench with the gun supported.
Most cap and ball guns shoot high.
Shooting from a supported postion will show if you are jerking the trigger when firing.
If the gun still shoots low from the rest then it's a easy fix.
Just file a little bit at a time from the front sight, checking after each filing for bullet impact.
You can always take more off ,but it's hard putting it back on once it's off, so check often.
Willy