Sam Hauser lacks agility. He's an ok defender because he has learned to stay in front of his man, but that's not enough. He appears to be a standing shooter for the same reason, just lack of agility. Frankly, I don't think that is ever going to change. This is his plateau.
With Stevens, Brissett, Myhailiuk and even Banton, plus Horford coming off the bench, I would say Hauser has a lot of competition this season to keep the "backup big wing" spot.