Farmers plant crops for one season and then stop

Most often this is because you have not set up a full rotation. Ensure crops/maintenance are assigned to each of the 3 rotations.

Note: The rotations in the UI also... rotate! This means that when the current year ends, the rotation moves to rotation 3 and rotation 2 becomes the current year (i.e., year 1). This is why it may seem like your farmers are only planting 1 rotation.