When problem solving, easiest way is to isolate 1 variable at a time. I'd just give it another austenitizing round and make sure the oil is in correct temp range for quenchI use a Hell's Forge propane forge; I know, not the best option, bit I didn't do enough research before I bought one.
Still, I've successfully heat treated 7 knives with it, and my more recent ones hold a pretty good edge and are quite tough.