Hi Alain. Let's look at the most simple. Have you checked your configuration settings? Go to System --> System Configuration --> System Settings.

Check if you are filtering or not filtering your alerts.
Another thing to check is whether the device is being polled, and whether the Up/Down Alarm is enabled. Go to a device that you are not getting up/down alerts:
- Check if the polling date/time is recent. If not, then the device isn't getting polled. Verify SNMP settings.
- Go to Interfaces
- From the interface listing, check what the "Up/Down Alarm Filter" is set to.