This could be due to weak batteries in the handheld. Replace with new batteries.
Is the camera tube fully inserted to the handheld unit?
When connecting the camera tube to the handheld unit, make sure the keyed ends are properly aligned. Once they are lined up, press in firmly and then finger tighten the knob to hold the connector in place.
Rotate the handheld unit’s Power On/LED Brightness control to the "ON" position and continue to increase the camera’s LED lights.
Models with wireless operation: make sure that the monitor is on Channel 4. If not, press the down arrow to change channels.