I have a DSC Alexor alarm system. I have two motion sensors (WS4904P) in my garage that are mounted 7' from the floor and tilted down slightly, and they don't seem to work. I can do jumping jacks around the garage and neither one will set the alarm off. They are configured properly as far as I can tell. The light on the front of them turn on from time to time and the main panel/keyboard alerts me when the batteries need replaced. What could be wrong?