Here’s the TLDR cuz it’s clearly bugged after extensive testing and thousands of mobs + hundreds of chests not granting fragments. Just delete the fragments, loot more, buy deleted ones back.
Delete a fragment of the type you’re no longer getting and let it sit in the vendor. Then continue your grind. DO NOT LOG OUT OR SELL ANY ITEMS SO YOU DON’T PERMA LOSE YOUR SOLD FRAGMENTS. Once you’ve looted your fresh new fragment(s), buy back your other fragment(s) and create your finished stone.
For some stupid reason the item drops can get bugged at 4/5 Thunderous or 5/7 Torrential. My friend had this issue and when we had him delete some and sit them in the vendor he immediately was able to finish. For whatever reason there was a hard cap and even looting 200+ chests and thousands of mobs didn’t get him anything until he sold some off.
GL HF enjoy