They may need their batteries replaced.

If any form of battery failure is detected, the red LED light will flash and the unit will “chirp” one time, followed by the warning message “LOW BATTERY.” This cycle will occur once every minute and will continue for at least seven days. To replace the battery, you must first remove the alarm from the mounting bracket. After the alarm has been removed, you can open the battery door and replace the battery. When installing the battery, press the battery reminder finger down into the battery compartment and install the battery. When you replace the battery you must make sure you push the reset button on the unit after you change the battery. After installing or changing the battery, reinstall your alarm. Test your alarm by using the test/reset button and check that the green LED light is on.

A good safety measure is to replace the batteries at least once every 6 months, or at the same time you change your clocks for daylight savings time. All of the batteries need to be changed at the same time since they are hard wired together. If one battery is weaker than another unit, it will pull from the other detectors and make each other go off.