You have launched an EC2 instance with four (4) 500 GB EBS Provisioned IOPS volumes attached. The EC2 instance is EBS-Optimized and supports 500 Mbps throughput...

You have launched an EC2 instance with four (4) 500 GB EBS Provisioned IOPS volumes attached. The EC2 instance is EBS-Optimized and supports 500 Mbps throughput between EC2 and EBS. The four EBS volumes are configured as a single RAID 0 device, and each Provisioned IOPS volume is provisioned with 4,000IOPS (4,000 16KB reads or writes), for a total of 16,000 random IOPS on the instance. The EC2 instance initially delivers the expected 16,000 IOPS random read and write performance. Sometime later, in order to increase the total random I/O performance of the instance, you add an additional two 500 GB EBS ProvisionedIOPS volumes to the RAID. Each volume is provisioned to 4,000 IOPs like the original four, for a total of 24,000 IOPS on the EC2 instance. Monitoring shows that the EC2 instance CPU utilization increased from 50% to 70%, but the total random IOPS measured at the instance level does not increase at all.
What is the problem and a valid solution?