need help quick please

[ammo burner]

I have an encore 50 cal muzzleloader. I shot it getting ready for my kansas hunt , which I am leaving for on the 25th of sept., and it shot well. 3'' at 200yrds. not bad. Cleaned it and shot it again and the bullets seemed to go down the barrel real easy. these were the same box that was shooting good. some of the shots had no power they had no recoil they felt like there was less than half a charge loaded. i am shooting american pioneer 50 gr sticks 150 grs with a 250 gr t/c shockwave. same powder and bullets were working good out of a prohunter and my knight. it seems that the pressure is not building because of the seal with the sabot to the barrel is weak. what could have happened between cleanings. been shooting this load for years and it has worked well out of all our guns.