Small Appliance Repair Questions? Ask a Technician ASAP
Try washing out the tank with warm water - shake vigorously to dislodge any stuck pieces of dirt, and empty the water through all of the tank openings.
If the problem persists, check that the filters and tank are correctly fitted.
Let me know if that works...
If that did not work then the iRobot likely has a faulty sensor and it will need to be sent for repair.