Sounds like your machines haven't been configured with the local scheduler task to send a scan on ip address change.
Take a look at your agent configuration and make sure that option has been selected. You can also check on the clients by running localsch.exe /tasks > tasks.txt from a command prompt and opening the file in notepad. Make sure to run this from the ldclient directory.
Also, just in case, make sure your inventory service has not stopped.
Mark Star - http://www.marxtar.com
Home of Power State Notifier & Wake-On-WAN for LANDesk