This would always happen to us and we'd always ask the dealer to put it back on. It would always keep on happening though BUT finally (after 6 years) an honest young man finally told us that its actually useless (then why would VW put it on the car i have no idea). If you have been in a VW beetle, you know how the front seats spring forward so that people can get into the back seats....well thats what that spring is for. But the jetta seats dont even do that because their is a back door for the rear passenger seats.
So the guy didnt even bother putting it back on (wherever it came from) and said to throw it out if we wanted....hmm kind of skeptical but what he told us made sense. And honestly we've gone times with the spring off and the seats feel the same so I just threw it in my garage just in case.
I have the same problem!! I've tried looking at the drivers seat, and can't even find a spring. Very strange, plus it's not easy to get a good look at what under there. All I see is all the electronics for the heated seats.