I know what you are on about, it's nothing to do with stiff seats!
My passenger seat has just had this problem and I can barely find anything on the net, someone here mentioned a snapped cable which is right.
This is what I did to find out exactly what is wrong:
- Raise both seats to the maximum height
- Compare both mechanism movements for driver and passenger seats to see what the passenger seat is not doing
- If the lever is moving down beneath the seat (long black metal thing) but not to the bottom it means it has weakened, if the thing doesn't move at all unless you do it manually then the cable connecting the lever (the thing you lift on top of your seat to move it) has snapped somewhere inside.
I am going to try and see if I can fix it but before I do I will get a price from a VAG specialist, cba with dealer prices.