Wave of Light and Obsidian interaction seems to be bugged on current PTR patch.
While hitting less then 5 (maybe only single target, not sure here) mobs the Obsidian proccs happen very infrequently, not on every attack as it should / did before.
It does not seem to be related to having Kyoshiros Blade equipped.
As soon as the pull is big enough, there is no problem maintaining cooldowns with Obsidian proccs.