Has the alarm been damaged?
If the alarm fails to trigger when your child wets the bed the alarm may have been damaged.
If your alarm rattles when it is shaken this indicates that internal componentry has been damaged. In this case a replacement alarm will need to be purchased.
If you open the battery holder and there are rust marks or white oxidisation on or around the battery this means the alarm has been subject to fluid ingress. Any liquid that gets inside the alarm will damage the electronic componentry and cause the alarm to fail. In this case a replacement alarm will need to be purchased.