Several things.
Order in some blackpowder from Powder Inc. Powder Inc will ship you 5 lbs. Here is their website.
http://www.powderinc.com/ Order Fg or Cannon grade. Don't get a heart attack when you see Powder Inc's price. Their price includes shipping and hazmat fee. Maybe some one like Zulu can tell you a source for Blackpowder in the Houston area...I think he is in Kingwood.
Do not use black powder substitutes in large bore cannons. Why? All these substitutes in rifle and pistol use generate higher pressures than equivalent charges of black powder. Some of them generate substantially higher pressures. The substitute formulas are for powders small arms quantities.
The max load charts for cannons are for black powder, not substitutes. I am unaware of any testing done to establish safe load tables for substitutes in the volume of powder used in cannons.
Most of the substitute black powders require resistance-pull, like from a tight patched ball and crimp of a cartridge to work correctly. When you use a bowling ball, the weight of the ball is giving you enough resistance allow the substitute to function. The golf ball does not provide enough resistance to be reliable. You have found that out.
Get a mortar made to shoot golf balls. When you put the golf ball in the bowling ball powder chamber, the chamber changes from being a mortar chamber to a golf ball cannon-the bore and powder chamber are the same diameter. If you use a bowling ball charge you are way over charging. If you use a smaller golf ball size charge it will just lay in the bottom of the bore and not burn right when ignited especially black powder substitutes. What you describe, sounds exactly like how black powder substitute act in cannons.
Don't use FFFFg is way to fast a burning powder for cannon. It is a priming powder only.
If you built a mortar like the Florida mortar post pictures. I don't recall telling you to not post pictures of your build. There were some issues with your powder chamber that concerned me but that was about it.
