In my instance, I needed to have two .iso images attached as optical devices during an install. I found that after adding the second image to the IDE controller, VirtualBox ceased to recognize the first image as bootable.
After much trial and error, I was able to boot once again by choosing Settings > Storage, selecting the bootable .iso image and changing its attributes to IDE Primary Slave.
For what it's worth, I also had to change the attributes of the other .iso image device to IDE Secondary before the Primary Slave option was available for the bootable image. I imagine the order in which you add the images to the storage tree may determine which attributes are assigned to them.