Here are the suggestions I received at the time from Fractal. I'm assuming it's okay to repost here since it doesn't involve opening the unit or anything radical that would void the warranty:
"1) Press and hold the switch down firmly (for a couple of seconds) while applying a steady force to the top of the black, plastic switch cover.
2) Repeat Step (1) several times allowing the switch a moment to return to its original position.
3) Rapidly press the switch several times using less force until it becomes responsive and is able to actuate freely.
Due to the accumulation of dimensional manufacturing tolerances along with the potential ingress of dust and small particulate into the rectangular bezel opening that surrounds the plastic switch cover, there can be enough of an interference to temporarily cause the actuation of the switch to be affected. Normally, the above procedure will remedy this problem."
Like I said, this didn't initially fix my button but it eventually popped out by itself, so perhaps it did help!