You have a bad button. If your top case is cracked along the edges, you should be able to get a replacement top case for free (since this model is currently less than five years old).
I believe I've read that there is a screw of some sort under the trackpad that makes it so one can't click. You can try looking at it yourself or take it to a local Apple store if that's an option.
Another possibility is a swollen battery, though. That'll cause the clicker to be stuck down. In this case, you wouldn't feel the click motion anymore.