1. Check the alert logic - is there something that may be excluding those devices? Perhaps a custom property that your Node Down alert is checking for that doesn't match to your device? Post a screenshot of your alert logic and we can assist in reviewing.
2. Confirm that your alert is setup correctly for sending the alert to Remedy. Are you using SMTP to forward the alert or a form of integration? What is the logic of the alert action? Another SS would be helpful.
3. A lot of people use a custom property to filter devices from alerting when they're undergoing maintenance - do you have something like this setup? if so, what is the current status of the property? is it toggled to mute alerts for the affected devices?
4. Alert suppression - are there any Alert Suppression rules that may be countering the trigger?
5. Time of Day restrictions that are preventing the trigger from executing?
Any info you can provide on the alert logic can hopefully help us diagnose this issue. If there are any custom properties referenced in the "Node Down" alert, please share an example of the current state of that property on one of the affected nodes.
If you feel the alert is configured correctly and is simply not executing when it should be, test and confirm that any alert is triggering properly. If none are, reset the services on the Solarwinds server.
There's also the option of creating a ticket with Solarwinds to aid in diagnosing and resolving this issue.