Is this the same breechplug you have been using all along? If so, you might need a new one. In addition to what is recommended above, I would suggest you thoroughly clean that barrel with a solvent that will remove plastic fouling. Even though you aren't shooting BH209 powder, a 1/8" drill bit in the breechplug's flame channel might remove fouling you are missing.

Inspect the internal crown as best you can to see if there are any distortions at all.