I collected 4 of the 5 leads very quickly and easily, in this specific order.
1) Petrified Oak Loop (Murkmire)
2) Stone Shank Frame (Glenumbra)
3) Sacred Resin (Malabal Tor)
4) Igneous Inlays (High Isle)
I then scried for and excavated those 4 leads BEFORE trying for the 5th lead from Titanclaw (World Boss). Over the past 3 days, I've killed Titanclaw literally no less than 50 times (and I'll continue trying although, with less fervor now), with multiple characters, always doing enough damage to have items available for looting from the corpse. Yet, despite this, the lead fails to drop. I truly believe the way I went about collecting and/or scrying then excavating the first 4 leads, has caused the entire lead chain to break in such a way that the 5th lead simply will not drop. Perhaps the way I went about it, bugged the game into believing I already received the lead so, it won't spawn for real? I'd appreciate this possibility being looked into. Thank you.