Sensors don't appear to be working

It has been a while since I installed my system, so I don’t remember much about how it works. I just went and walked by all my different motion sensors to test the alarm, and two of them did not appear to set off the alarm even when I was waving my arms frantically in front of them. Is this batteries? How would I know if it is batteries before they actually run out so I don’t just have an unprotected zone of the house. If so, how do I change them out?

Motions can be frustrating to test. They do require some patience that other sensors do not require.
They will not trip immediately if recent motion has occurred in front of them without a rest period. As a battery saving function, the motion detectors will need a reset period. Typically it is recommended to arm, wait 3 minutes outside of the detector view, then walk in front of the detector.
Waving your arms in front should be totally unnecessary. Walking by within range should trip the alarm.

If batteries are low, your system and account will report this on a per sensor basis. Once in a while, a battery might be bad however and perhaps discharge too quickly. If you get no result from the alarm test or sensor test, you can try putting new batteries in.