Don't know if they make a BH 209 specific breech plug for the Encore, but I use the standard one in mine with no troubles. Try to press the bullet down onto the powder charge with a fair amount of force. I've read 209 needs to be packed pretty firmly to ensure consistent ignition. You might also try Federal 209 primers; I think that's what Blackhorn recommends.
That's definitely good shooting, both on your part and the gun's. That setup ought to get you past 100 yards.