Note that the spacebar has four metal brackets/bars. The inner two can stay in the key itself. But the ends of the outer two must hook underneath four little clasps that are on the base of the keyboard. If not, the far ends won't work (underneath the C and M keys).
So if you see all four metal brackets when you pull off the spacebar, gently take off the outer two and hook them into the clasps on the computer itself. Then line everything up and snap it together.