I have a hero track tier helm on my paladin 619 4/6 upgraded. I believe it is a catalyst item that I got for completing a weekly timewalking quest to kill Illidan.
I purchased the Nerubian Gemcutter with 6 coins from my vault and I cannot use the gem socket item on this helm. I am able to use it on other helms, just not this one.
I believe this to be a bug.
If that is not a helmet that falls under this support article
specificially “Our developers have fixed an issue which resulted in the Nerubian Gemweaver not working on Timewalking raid and dungeon items obtained at level 80. However, this fix may not be retroactive. Customer Support is unable to assist players affected by this issue.” (emphasis mine), then you should write an in-game bug report. Nobody reads the forums for bug reports in Europe, it’s all done with the in-game reporting tool.