Provided in one example is a printing system. The system includes a printing zone including a platen (50). The system includes a drive to move a medium (58) through the printing zone. The system includes a suction force generator (78) to generate a suction force to hold down the medium onto the platen while the drive moves the medium through the printing zone. The system includes a sensor (52) to monitor the movement of the medium through the printing zone in a medium advance direction. The system includes a controller to determine that the suction force generated by the suction source generator is insufficient based on an output of the sensor.