Seems to be a mixture of a few things...
1. Some bad puck luck. He's had a ton of missed shots/posts/crossbars/would-be goals hitting knobs of goalie sticks this season, often during some of his best chances, leading to an unsustainably low conversion rate so far.
2. No free or easy goals. All of his ES goals are 5v5. No extra ES goals 3v3, 4v4, 6v5, or into an empty net. Rantanen, for example, has a league-leading 21 ES goals, but only 14 are 5v5. Ovechkin has 20 ES goals, but only 12 are 5v5.
3. He's being shadowed and double teamed like crazy after last year's MVP season. No time or space to do anything but quickly pass a lot this year.
4. Perhaps as a result of #3, or perhaps just an attempt to expand his skillset, he seems to be focusing more on playmaking this year.
5. Quite frankly, he doesn't mesh well with Nylander. They play a different style of game, and now that Marner isn't there, Matthews has to take on pretty much all of the defensive responsibility of the line.
I wouldn't worry too much. The line is still producing well, and an underrated amount of that is because of Matthews, even if he's not the one getting the goals (or an abnormal amount of times, even the points). He's doing so much of the work and opening up so much space and opportunity for Nylander/Bunting to do their thing.
It was funny seeing people run to declare Nylander our MVP because he had like 1 more point over less than half a season, in part because of all the uncredited work Matthews was putting in. Personally, I'm encouraged by how committed Matthews is to team over individual, and I'm sure Matthews will hit a hot stretch at some point.