Both above good advice. You may even want to consider using a plastic shot wad/cup to tighten up the pattern even more.
One reason that your pattern may have been tighter with 2 wads (and I'm assuming here) is that the extra wad cushioned the shot pellets. Deformed pellets can fly off in all directions. That's where a modern shot cup would help out.
I'm just thinking out loud. I never owned a BP shotgun but I have reloaded shotgun shells.