Have you removed the key and checked it is seated properly? Also, is the little metal clip that runs underneath the key in the correct slots? If the clip is off, the space key will work fine if you press in the center, but wobble and be hard to press if you press off center.
Another thing to check for is gunk in the plastic pieces. People do dumb thing like eat or drink over their keyboard, and over time food and liquids can get stuck in these pieces. There are several videos on YouTube that show you how to remove keys and clean these pieces.