First and foremost, Eichel is holding himself back IMO. This may strange, but when I watch him play, it just looks like he's playing with the attitude of "I don't want to show off and appear to be better than everybody else". It's that same attitude you see with guys in Beer Leagues that are quite obviously too good for that particular league but want to get a skate in.
Honestly, he's a guy that constantly needs to be pushed and motivated to be better. Even if you have to tell him, "Eichel, you're not that good" in hopes of him getting the motivation to prove you wrong, you have to kind of give him a wake up call. I would love for him to kick his training up a notch, and not just focus on the training in the weight room, but raise his level of training. Go train with the McDavid's, Marchand's, and MacKinnon's of the league. More time training in tight, against elite players.
IMO, Eichel could be an easy 2 PPG player (I've said 4 PPG before and still think so). He needs to tweak his mindset and the areas he tries to play in on the ice. Make guys take penalties on you. He's as strong as anybody in the league, he's faster than mostly everybody in the league. If he's not making it hard for guys to stop him going to the net/crease, then Jack is not trying hard enough.
There are other areas of the team than can have an impact on his numbers, but at the end of the day, Jack can only control his actions and his attitude. It's on him to get better.