I've been setting up a few Cloudwatch sensors, all with the same AWS user (access and private key), and that works fine.
However, when presented with the list of EC2 instances and EBS volumes to pick the item to be monitored, it lists all except for one of my EC2 instances. It's that particular instance I really need to monitor (of course...).
I've done a bit of investigation by using different access keys (for users with more rights) but get the same result. I've then used the AWS CLI to list the EC2 instances, and found one interesting fact: this server is the last one in the list returned by the CLI when using the describe-instances command. That made me think it could have something to do with how the list is parsed in PRTG; and that's the only clue I have.
Everything else about this server is identical to some of the other servers that are listed fine: subnets, VPC, region, Security group - it's all identical...