Forklift drivers fall asleep when their manager level is too low.
To keep them awake, upgrade your forklift manager. Higher sawmills require higher manager levels to automate forklifts, so upgrading early helps avoid downtime.
 
Automate the forklift
You can also check if your drivers are sleeping by opening the Order Status window.
If they are asleep, you will see a special button there.
Tap it to wake them up and get your forklifts moving again.