Was playing around with using the loopback devices to mount some OS images on CentOS and following the losetup followed by a mount command would indeed mount the image, but umount wouldn’t unmount it and would leave it stagnant, leaving me with this error:

Interestingly enough though the lomount command

and then using umount to unmount the mountfolder would unmount the loopback device properly and free it for re-use.

Hope this helps someone!