From the Angelcare Monitor User Guide:

Adjusting the Movement Sensitivity.

IMPORTANT! The sensitivity level has been preset. DO NOT adjust unless necessary (oops!)

The sensitivity adjustment dial has been preset to suit most situations. However in certain circumstances, the sensitivity may not be ideal. DO NOT adjust the sensitivity level unless you have followed all the advice in the troubleshooting section I'll put this bit at the end If you do change the sensitivity level, make a note of the preset level before adjustment mine was 3. To adjust the sensitivity, increase the dial to the next setting. Retest monitor.

In the troubleshooting bit it says that the pad must be on a flat surface and if there's slats or sprung - to place a piece of MDF underneath it to stabilise it. It also says to check the pad(s) are plugged in properly Otherwise - adjust the sensitivity.

keep in mind that when you adjust the sensitivity and test the alarm - if you stand next to the cot - your breathing might be enough to register on the pads. Move right away from the bed when you test it. If you have a ceiling fan (or other fan) - that too might be enough to keep the pads registering. It took a few tests to get the setting right for us. It will still go off sometimes if DS has moved to the end of the cot.

Hope this helps!