First, make sure no dirt or debris is blocking the drop sensors on the bottom of the robot. Wipe the sensors with a clean, soft cloth or cotton swab: clean the dirt from the windows of the sensor. Start the robot again and continue cleaning.
If the robot continues to have trouble with stairs, you can lay boundary markers or setup No-Go Lines on compatible models to prevent the robot fro going near the stairs, especially if the stairs are covered with thick carpeting. Thick carpeting can affect the robot's navigation system. We recommend using removable tape such as painters tape to keep the boundary market low on the carpet.
We recommend cleaning the drop sensors regularly: how often depends on the amount of dirt and debris in your home. For example, if you have pets, we recommend cleaning the drop sensors at least once a month.