A few things to check
- You have a long list of routers in the complex "any" condition. I am assuming the router in question is in that list, and you have verified that the router name displayed in the node details view matches exactly with the value you have used for the filtering condition
- The alert also filters on a custom property Mute = No. I am assuming you have checked the custom property assignment for this router (i.e., it is set to No)
- The default 'Alert me when a node goes down' has been disabled
Assuming you have already checked the above, there is no reason why the alert engine would not send an email just for this router and work fine for others. It is best to grab diagnostics, and have support look at it again.
With that said, this alert is a difficult one for your team to maintain. I would suggest that you create a custom property called NetworkDeviceType or something similar and create a drop down list such as Routers, Switches, Firewalls, etc. Set this custom property to Routers for all these routers. You alert will look far cleaner when you do that, and far easier to maintain. The trigger condition would be
Node Status is equal to Down
Mute is equal to No
NetworkDeviceType is equal to Routers