I would be willing to bet that it is the white nylon gear is broken. This is the exact same thing that happened to me. I would engage the button to recline the passenger seat and could hear the motor run, but the seatback would not move. I took the seat out, removed the rear motors and took the cover off a small gearbox. I discovered a small white nylon gear was broken. The BMW dealer does not sell a replacement gear and could not find one in a salvage yard. Luckily, I posted my problem on another message board and someone responded. He took the initative and had new gears custom machined out of bronze. The cost was about $80 and the fit was perfect. Also, I will never have to worry about the gear failing again. However, there are a total of 4 of these nylon gears (2 in each seat). I would suggest removing the good gear and contacting a local machinist to see if they can duplicate the gear.