Motion Detector not working

Hi all. Looking for any advice. Installed the alarm system and all was well. Several hours later added three new users (my kids) who will need access to set and disarm the system. The motion detector then failed to trigger the alarm, although it reports motion detected. It was set up in Room with 30 sec delay. Nothing I could do would get it to trigger the alarm. So, removed it from the system. Now, it will not connect. Have removed and replaced battery but no luck. Anyone else had this issue? Really frustrating as it was working perfectly, for all of two hours! Thanks in advance.