Kalista's W is not going off with her Oathsworn

Kalista and her Oathsworn'd teammate are unable to proc W. Only on very rare occasions if you hit two minions within a few seconds of each other will it register that it's been AA'd by the oathsworn, likewise then it is almost impossible to proc the W onto an enemy champion because it will not go off at first basic attack.

Has anyone else seen this issue? Please help to investigate.