This is usually because the metal stabilizer bar or the plastic pieces need to be lubricated. You can lubricate the key yourself by following these tips:
• Using a key puller carefully release the key cap, but you don’t want to pull it all the way off. The large keys are hooked to a stabilizer bar. They are tricky to get off, and even trickier to put back on. You only need to loosen the key cap enough to rock it to one side so you can lubricate the metal or plastic pieces. It's helpful to remove the keys immediately surrounding it to allow easier access.
• Apply a small amount of lubricant or oil to the stabilizer bar, being careful not to touch the key switch.
• Snap the key cap back in place, and you should be all set.
We have a video on how to do this: http://www.daskeyboard.com/blog/?p=3212
If you do not have a key puller please advise your address, and I will send you one (U.S. only).
We do not recommend using WD-40 or other petroleum-based solvents as they can cause damage to plastic over time. White Lithium Grease or Super Lube (a synthetic and safe for plastics) is recommended.