So, I've had troubles with
add_theme_support('post-thumbnails') in the past, but this is entirely new.
My team is working on a project and the meta box to add a featured image only shows up on one team member's working copy. He's on a windows machine, whereas the rest of us are on macs.
I've disabled all plugins, and enabled the Twenty Eleven theme. It still doesn't work.
If I do it on another project (disabling plugins and enabling Twenty Eleven), still locally, it works perfectly...
If anyone has any clue what's going on here, assistance is much appreciated.