Why won’t my CloudWatch billing alert trigger, even after crossing threshold?

There are several potential reasons why your CloudWatch billing alert isn't triggering despite crossing the threshold:

1. Billing alerts not enabled: You need to first enable billing alerts in the AWS Billing and Cost Management console. Navigate to Billing preferences, edit Alert preferences, and select "Receive CloudWatch billing alerts."

2. Region issue: Billing metrics are only available in the US East (N. Virginia) region. Make sure you've created your CloudWatch billing alarm in this specific region, regardless of where your resources are located. If you're trying to create the alarm with another region open in the console, you might see an error about metrics from different regions.

3. Evaluation period: CloudWatch alarms evaluate metrics based on data points available at the time of evaluation. Standard alarms evaluate every minute, and the billing metric typically updates every 6 hours. If you've just set up the alarm, it might need more time to collect sufficient data points.

4. Alarm configuration: Check your alarm configuration, particularly:

• Statistic: Should be set to "Maximum" for billing alarms
• Period: Typically 6 hours for billing metrics
• Missing data treatment: Should be set to "Treat missing data as missing"
• Datapoints to alarm: Should be "1 out of 1" for a simple threshold breach

5. Notification configuration: Verify that your SNS topic is properly configured and that you've confirmed any subscription emails that were sent when you set up the topic.

If all these settings are correct, you might need to wait longer for the billing data to update, as it's not a real-time metric and can take several hours to reflect changes in your AWS usage.
Sources
Troubleshoot a CloudWatch alarm that doesn't invoke | AWS re:Post
Create a billing alarm to monitor your estimated AWS charges - Amazon CloudWatch
Unable to create Billing alarms in CloudWatch | AWS re:Post
Customizing your Billing preferences - AWS Billing