Yea, it can take several failed attempts just to loosen up the material to prepare it for release,
I have completed over 70 flushes, and there were times where I would flush ten times in a row and not yield one stone, and then the next flush yielded 100 massive one.
The stones sit in the liver for so long that they become ingrained into the liver tissue, so it can take several pushes to loosen them up to come out.
If you don't yield anything with the coffee enema itself, try a coffee enema followed with an oil flush.