killing palm sometimes does not activate at all

On my second character Monk, i observed in Ogham manor on the final bossfight that sometimes killing palm does not work at all. I did now second time in a row on the final boss fight with the boss glowing ready for a culling strike with killing palm, my mouse cursor is on him and it does not activate. I am spamming the button and nothing happens.

We dance around for about 10s while i try to kill him with the culling strike from killing palm but nothing happens until i die.

That happened two times on this boss alone. But i observered that previously several times. I wasn't sure earlier as the situations where with weaker opponents so they died anyway. I thought i did something wrong.

But having now a boss ready to be culled for 10 seconds and me not being able to pull off killing palm for such a long time is annoying.