Alerts not clearing

In my active alerts, I have a server that is saying that the databases have not been backed up in XXX days.
I've resolved the problem, but the alerts are not clearing.
When I go and look at the database backup data, the "Last Backup" data is correct (and under the alert level)

What can I do to clear the resolved alerts?

Note: This is happening for more than one server, but not all of them.