What am I doing wrong with the fuel and packing I am using for a cannon I made?
Flash powder and gun powder are two different mixtures, designed to do two different things. Gun powder is designed to produce a lot of gas very quickly (Carbon dioxide, Sulfur dioxide, Nitrogen) which propels the projectile out of the barrel.

Flash powder - typically potassium chlorate/perchlorate and aluminium powder, examples here and here, produce a lot of heat and light very quickly but not much gas - so no propulsion.