The melee ability for Warlock's Stormcaller middle tree, "Ball Lightning", no longer procs with the exotic Crown of Tempests, despite being an arc ability. CoT has the exotic perk "Conduction Tines", which activates on arc ability kills. All arc grenades, supers, and all other arc melees can activate this buff. Ball Lightning used to be able to activate the perk before the recent update, but hasn't since. If I had to guess what is happening, I would say that the changes to Ball Lightning in the recent patch (that allow it to proc Trench Barrel at close range) has made the game consider Ball Lightning a regular melee attack instead of a ranged arc ability.