Because of how the Feed Mill mastery stars are set up, it is harder to gain a star compared to most of the other production buildings.
To gain one star in the Feed Mill, you must produce goods for a total of 120 hours, compared to only 35 hours in the Bakery or 40 hours in the Dairy. Like with any production machine in the game, after you have reached the first star, the counter will start again at 0 (in case of the Feed Mill, this would be 0 out of 840 hours to reach the second star).
Another playing factor of why it takes longer to reach stars in the Feed Mill is that the production time of produce made in the Feed Mill is very short. Chicken feed, for example, takes 5 minutes to be produced. Meaning that it will take 12 chicken feeds to gain one hour on the mastery points.
It may seem difficult but it is set up that way to create a nice challenge for players. 😉