I had a problem a while ago with one side of the rear seat folding. The head rest would not fold back so I couldn't get the seat folded right down. I fiddled about with it for a while but couldn't find any problems and then one day it magically started working OK so I just forgot about it.
A couple of weeks ago I started getting the same problem with the other side.
I had a few minutes spare so started mucking around to find out what was causing this. It seems that the head rest is unlocked when the seat back tips beyond a certain point. The passenger seat was further back than the driver's seat so the head rest hit this before reaching the unlock point.
If you get a problem with you head rests not folding back, shunt the front seat forward and ensure the back is in the upright position. After doing this the rear seat folded forward and I heard the release click so the the head rest folded neatly.