Try No 11's they are slightly bigger.
Do you have this problem after the first few shots?
If it is a new gun have you had it apart to make sure it is not gummed up with grease from the factory causing the hammer to drag?
It is comon practice with a muzzloader to "cap off". That is fire a cap on every chamber to make sure that any oil left in the gun is burnt off before you load it.
Hope this helps