You could say years, because this is not a problem that just popped up last season. It's more like this is an ongoing problem that bottomed out last season. Part of it is still not having replaced Nick Leddy as a puck carrier, the other part is the continued use of the same players...Barzal, Lee, Nelson, Pulock (Bailey is finally off the roster otherwise he'd be on the list too), who either hold the puck too long, can't pass well, don't headman up the ice, don't skate well, don't hit the net regularly. In short don't execute well enough. Guys who really dont' possess enough of the tools to fill the toolbox they need to run a power play. Until this changes it doesn't matter who coaches the power play.
Addendum: I think the reason we can be good at 5 v 5 and so poor on the power play is because the Islanders generate offense at 5v5 off the forecheck where board work and physical play are their strengths. The power play requires quick puck movement, execution and some measure of finesse. This is not our best attribute.