Unable to suspend on MacBook 11,1 on Ubuntu 14.04

I just finished installing Linux Mint 17.1 on my MacBook 11,2 (2013 model, 13.3") and have just wrapped up fixing most of the problems.

One problem remains: I can't suspend (and by consequence, resume) the machine. If I close the laptop lid, it just locks the screen, no suspend. If I click the main menu and hit suspend, it locks the screen, no suspend.

I'm not sure where to get started on figuring out what the problem is.

How can I start debugging what's wrong so I can fix it?