I decided to update AE to try and fix the issue, (not sure which version I was running, my latest update was probably just a couple months ago). I'm now using version 184.108.40.206
Try updating to the latest version of CC 2015. If you're going to use AE CC 2015, you might as well use the latest one. The latest is 13.7.0.
You could also consider using CC 2014 (AE version 13.2). A lot of people consider CC 2015 (AE version 13.7) a beta product. That's because AE is being completely redone!
AE CC 2015 is the first step in a major re-architecture, separating the renderer from the UI for the first time in AE's 20+ year history. This is a big job and it's clearly still in progress. It's a necessary step for improving After Effects performance in the future, but it's taking a while (and multiple versions) for them to get through it. So, during this time of transition as they're introducing elements of the new architecture, sometimes it may be easier to use the older version to get a job done.
You can open projects created in AE CC 2015 in AE CC 2014 with no problems.
That's strange, I have updated it though, I literally made that post 5 minutes after the update when I found out it still wasn't working. I have no option to update again, so I am to assume 13.5 is the current version :/
Perhaps restarting the mac would show some later versions, weird...