I have a stone that I found when I was still quite new to wet shaving. It looked like a thurinigian, slurried like a thuringian, soft like a thuringian but when I used it after an 8k I could not improve the edge. For the longest time I figured it was a 4-6k stone. Not until recently I revisited this stone and was able to polish the edge to a very very fine level. I found that it was a slower stone, not quite as quick as 2 other thuris I have, but very good once I figured it out.
Don't disregard it as a good stone yet, give it some time and mess around with it a bit. Less slurry, more slurry, no slurry.