I felt like you when my stones just kept coming and they didn't appear to any end in sight.
I concluded that as you suggest the liver was very congested. I also worked out that a gallbladder can suffer from stasis, meaning that it doesn't contract fully which is logical if it is full of stones. Each flush will improve the function of the gallbladder until it reaches its maximum efficiency. This event happened with me on flush 20 and I am currently getting rid of a few stranglers but I consider myself clear.
I did increase the dose of the potion gradually to make the biliary system work as hard as possible.