Well, for one thing, featuring Lehtera and Filppula as the forwards on your second unit is a recipe for disaster. Those guys lack to quickness to close down passing/shooting lanes at even strength, let alone with one less guy on the ice. Hextall and Hakstol seem hellbent on developing Laughton into this defensively responsible fourth liner, but refuse to give him an extended look in a PK role.
The combination of Hagg, MacDonald, and Manning get this reputation of being defensively responsible, but they're all routinely behind the play and late in getting sticks tied up, shooting/passing lanes blocked up, etc. You can't just rely on your goaltender to be your best pk'er. Again, it comes down to a lack of foot speed/quickness and also a lack of hockey sense to make up for it.