Storage Migration Service Missing Source Volumes

Hello,

I'm trying out the Storage Migration Service for the first time. I've come across an issue where Storage Migration Service is unable to see some source volumes. For example, the cluster I'm migrating from has an F:\ but it is not visible on Volumes tab when performing a scan:

The strange thing is the volume is visible on shares tab:

When I continue to the transfer page, I'm unable to migrate the share because the source is not available. This problem affects around 3 of my volumes - all the others are fine and I can find no obvious difference between them. I'm absolutely stumped by this problem. Could this be some kind of bug? The source server is Windows 2012 R2 (two node windows file server cluster) - All windows updates are installed. The orchestrator server is Windows Server 2019. 

Thanks,

Henry