The first suggestion I found was to lubricate the plastic posts under the space bar to allow the key to move more smoothly. I used a tiny dab of vegetable cooking oil to lube up the plastic surfaces that make contact with each other on the key and body of the keyboard. Then I snapped the key back in place. It didn't make a difference at all. The key still felt stiff.

The next possible solution I came across suggested that the metal sway bar attached to the underside of the key needed to be "tweaked" or adjusted. I popped the key back out, set it down upside down on my desk, and twisted the ends of the sway bar in opposite directions. Once the ends were just about a fraction of an inch out of line with each other, I re-attached the space bar key and tested it out. The key sat slightly lopsided but the effort needed to press it was significantly reduced but only on one side. Tweaking the sway bar ends back in the opposite direction switched the "easy" and "stiff" sides. If you only use one thumb to hit the space bar, this could be a viable solution for your keyboard. I use both thumbs so the search continued for a better solution.

The last possible solution I found suggested that the metal sway bar should just be removed completely. I carefully popped the metal sway bar out of the plastic retaining clips on the underside of the spacebar key and put the key back on the keyboard. The space bar felt more "normal" like I was used to with my well worn white MS Natural Keyboard Elite but it just wasn't the same.

So I returned the Microsoft Natural Ergo Keyboard 4000 to the store and re-attached the MS Natural Keyboard Elite again. Sure this old keyboard has a hard plastic wrist rest, loud clacking keys, and a not so fashionable white finish but it works properly. *Sigh* I guess I'll just have to wait for the next generation of MS Natural Keyboards or switch to another brand if this one ever breaks.