Expand/extend the Amazon EBS root volume of my Amazon EC2 Windows instance

I've expanded the disk size of our EC2 r5.2xlarge Windows instance. This seemed to go fine, with the volume indicating the new size and the optimising process successfully completing with the Volume Status showing as "Ok". However, when I try to extend the disk in the Disk Management tool on our EC2 instance the new unallocated disk space does not appear, and the Extend Volume option is greyed out. While there is a second disk on the EC2 for storage the disk I'm trying to extend is not partitioned. I'm a bit stumped and would appreciate any advice. Thanks.