Practice makes perfect.
I've had minimal success with this technique until I learned how to use the pressure cooker properly. Only adding enough water so that the PC is almost dry when the 1 hour time is up. Previously, I was adding too much water to the PC because I didn't want it to run dry. Too much water inside the PC was adding water to the INSIDE of the jars. I could see the liquid at the bottom of the jar after the PC which wasn't there before PC.
Poured out wet popcorn, dried on towel overnight, re-PC and again too much water — did this 3 times, WTF I'm thinking...
Did some searching around a bit and found a posting by someone's Grandma about keeping her canned cherries as dry as possible. She mentioned making sure that the water is almost gone at the end of PC. Basically at 1 hour time the steam will begin to slow - that's when it's getting dry inside and time to stop. It does take some trial and error for your specific PC and number of jars inside
Anyway, jars come out dry, I inoculate and in 1 week I have fully colonized jars. Mycelium growth fucking EXPLODED!!
Now I know I'm doing g it right... Finally!