The usual reason for Guest Additions failing to install on a Linux guest on Virtual Box is that the kernel headers are missing. How you install these or where they come from varies from distro to distro, although they’re usually available via the package manager on that distro.
I had an Oracle Linux 6 guest installed, Guest Additions (for video drivers, shared folder, clipboard sharing) was all working, and then at some point I started it up again and it was no longer working and wouldn’t re-install either. Seems like I’d picked up an update, and I needed to update the kernel headers too.
This post covers the steps needed. On OE6 before installing the Guest Additions, just run ‘yum install kernel-uek-devel’ and you should be good to go (assuming you’re booting with the ‘unbreakable kernel’ and not the RHEL compatible kernel)