kha'zix Isolation

I recently have been playing Kha'zix a lot since the update and I notice the isolation passive doesn't always want to mark targets. Last night I was in a match and Viktor was on the river side of dragon pit and his team was by their red. I jumped over and attack and my Q did not reset, nor did it see him as isolated. I have experience this frequently as Kha'zix and was wondering if this is a bug.