Well, the trackpad assembly is fully immersed and melted in the topcase, so there's no way you could act in it.
You can try using a piece of paper or something thin enough and move between the button and the trackpad, to see of something got lodged that.
Or try the topcase without the battery: a chinese battery I have presses on the topcase for example, making the button "sturdier" at touch. If without battery the button works fine, that means the battery is pushing on the mechanism.