It's a combination coaching/depth issue. For most of the year, Mantha has been saddled with mediocre linemates (by AHL standards) that he doesn't mesh with. He was showing some promising chemistry with Andreas Athanasiou, but AA got injured and Mantha has been pretty quiet since.
But I say there's a coaching issue because there's a lot of round peg square hole type coaching going on. Mantha doesn't do well with linemates X and Y? Let's play him with them until he does! and so on.
In the games I've seen, he's been pretty good. He has flashes of crazy gamebreaking skill, and his skating is very good. He still floats a bit at times, and I think that's another reason why coach Blashill is giving him a hard time.