Depends what the problem is, I guess.
First, I'd suggest removing the key. (Carefully! It has a long metal bar underneath; probably best to leave that attached, and not lift the key too much.) Check it's all clean and nothing's stuck underneath.
If the key gets stuck hard, then check the plastic prongs underneath and see if they need shaving or anything. (Despite the longer key, I think the prongs &c are the same as they other keys.)
Or if the rubber mat lacks the power to lift the key after it's been pressed, Planet Computers might send you a replacement mat if you ask them. In any case, it's probably worth contacting them if you can't solve it yourself.
(Oddly enough, my space bar has exactly the opposite problem: unless I press it in the right place, it won't press down!)
