I wouldn't consider any flush to be an unsuccessful one, because even if nothing comes out, the force of bile pushing through the ducts helps to loosen up the congestion to make it easier for stones to be expelled on a future flush,
Yea, there is most likely a huge stone or compacted batch of small stones that have ingrained themselves so deeply into the tissue of the liver, that it is making it harder for the congestion behind it or them to come out.
I had the same problem as you are having, until I started double dosing with Gold Coin Grass in tincture form. I had at least ten to twelve flushes where absolutely nothing came out - I didn't even yield one stone.
I was taking lots of different things, but they were mostly in pill form, aside from the high doses of malic acid in warm tea I was drinking regularly.
I don't have a whole lot of confidence in pill forms, they don't really seem to absorb into the stones as well as tincture form.
Not only does tincture give the advantage of being in liquid form and being able to permeate the liver more thoroughly, but it comes mixed in a little bit of alcohol, causing the medicine to be directly transported into the bile ducts of the liver.
After taking GCG for just a week, I did another flush, and tons of gravel and broken up stones came out - it was incredible. I kept drinking the GCG tincture, and flushed again two weeks later, and released the batch of congestion that was causing most of my symptoms - the stones were an inch wide and very heavy.
If you are only releasing a few stones at a time, you won't really notice the results, and in that sense it is unsuccessful, but I can assure you that if the congestion is softened and or broken up properly, you will expel much more material, and begin to experience life changing benefits.
New Way Herbs is a US supplier, which I have found to be a little bit more expensive in product price, but shipping is cheap. I think the product may come in higher concentrations, which is why it is more expensive.