They have a drop chance of .05% meaning you'll get a single one every 2000 blocks. How much soul sand have you been digging?
So you have around 9200-11500 blocks, which should have yielded around 7 or so shards at 0.05% (if I did my math right). Did you happen to buy these inventories from spawn shops or have them laying around? If you placed them, McMMO drops aren't given I think, which might be the cause of the issue. Feel free to correct me if I am wrong.
Sounds like you either had really bad luck or it's not working. I can get in-game to check it out shortly.
You are the King of excavation then And I just made another run and did not get any crystals either with 2 inventory full of soul sand. Of course Kuke already has taken care of it - as usual.
Same result here, I just went out and dug 7500 soul sand and didn't get any crystals. Just to make sure. Also thanks kuke for helping.