Originally Posted by Sleepy Hollow

I did add ACV (but only a teaspoon)... The beef bones are just very thick. After 24 hours, a good bit of marrow finally released from one of the bones. I ended up adding just a little more ACV and hitting the ball joint with a hammer. It's softened up quite a bit and now the inner bone is more exposed. Leaving it on the slow cooker setting won't take any more effort, so I may as well leave it cooking longer.

Do you do the 4 hours on full high pressure? That may be part of the difference too. The recipe I used said to do pressure for 30 minutes, then warm for 20, repeat two more times, so only actively at high pressure for 90 minutes.