If you want space on all sides, you should be on the center anchor point. Show a screen shot if it's not working.
An alternate method to add canvas is to click the crop tool, and drag the canvas bigger. You can use Shift to keep it proportional and Option (Alt) to resize it from the center.
Try that if the conventional method is failing.
You are typing in numbers for both a new width and a new height, right? That would be the first thing to look at.
Thanks for responding Jane! I had the center anchor selected; turns out, after working with an adobe rep, the solution was resetting the tools. Why? I don't know. That's just how he managed to fix it.
If you were changing Canvas size with the Crop tool, that makes sense.
But it absolutely does not make sense that resetting the tools would fix it if you were going to Image menu > Canvas size, as that is not a tool, it's a menu item.
Resetting Preferences would also make sense if something had gone flakey.
I'm glad it's working, but I wonder if Resetting Tools actually solved it or if it was coincidence?
Anyway, at least you can move on with your project, so all is well!