Consider the following: Among the 16 teams that made the playoffs this season, and through the first handful of games in the postseason, Tampa ranks third last in CF/60 on the power play. They simply aren't throwing the puck towards the net with regularity. And, if you look at teams that have consistently strong power plays, you'll see that they typically send the puck in the opposing goaltender's direction with some measure of regularity. Tampa didn't do that in the regular season. Tampa hasn't done that for the most part so far in the postseason. A visual representation:
Obviously the small sample size makes the disparity far greater than it likely would be over time, but the fact remains that Tampa Bay has failed to generate shot attempts at the same rate as some of the teams they are competing against for hockey's biggest prize. It's just not a recipe for success if you're looking to score with the man advantage.