Might be related to this?
Tl;dr, some item effects are auras that were not being applied correctly or being overwritten by other buffs.
I saw another person posting about naglering not working as well and he was also a druid. I wonder if it’s conflicting with Thorns? Try removing thorns from your character and then unequipping and equipping naglering again and see if it works. I saw some one else post in the other thread saying it worked fine but he wasn’t a druid so maybe that’s where the bug comes from.
In any case, if it’s related to the issue I linked, it sounds like blizz has a fix on the way.