Weird VirtualBox issue: broken permissions on newly created files in shared folder

[Zinn]

I'm having an intermittent issue with my xubuntu installation on VirtualBox. I'm using the VM as a web server, so it is mounting a shared htdocs folder that lives in my Windows machine. Sometimes when I create a new file in Windows, the permissions are all f'ed up when trying to read via the VM. The file shows up, but with no permissions, owner or group. The file is unreadable from the VM. If I reboot the VM then the new file shows up appropriately and everything works. I can't reproduce the issue reliably--it happens with some files I create in Windows and not others.

In the example above, the "tools" folder was created in windows and is unreadable by the VM until after I reboot.

I'm not sure what I'm doing wrong. Any suggestions would be appreciated