You might look at your patterns of fat intake. Not eating enough fat in your normal diet can cause the flush to fail, just as eating fat during the pre-cleanse period can cause it to fail.
Coming off a juice fast, I didn't produce stones when liver flushing. The first time, I didn't think anything of it. The next flush did produce stones, but after another juice fast, I flushed without producing stones. That showed a pattern.
I hadn't had any oil or fat for three weeks. My liver had shut down bile production, so the flush was not effective. The epsom salts washed me out, but there wasn't enough bile to flush my liver.
Now, I add a tablespoon of gound flax seeds to my veggie juice, just before I drink it. This gives me fiber, a bit of protein and fat, so my digestion doesn't totally stop. My bowels appreciate it and the liver flush afterward is terrific!