Pull the barrel off the rifle if possible. Now take the breech plug out and dump the powder out. Now with a patch saturated with Windex, swab from the breech up to the bullet, and from the muzzle with another patch swab down to the projectile. That should remove all fowling. Now squirt a generous amount of gun oil down the barrel. If you have a long range rod, do it from the breech up. And also squirt some from the muzzle down. Let it sit a few minutes and with a wooden dowel or a long range rod, you should be able to push that thing out.

Switch to a thinner sabot. The same thing happens to me when I shoot a SST with their supplied sabot. The Shockwaves are fine. Also are you swabbing between shots? If one loaded and you swab, the next should load also... Good luck and keep us posted.