Yeah, your motor for the seat is either out, or the gears are stripped. It's an expensive fix. I cut mine loose, separated the motor from the seat frame, and then strategically placed a 4x4 block of wood under the seat bottom in the back to keep the seat up. The bottom drops to the floor when the seat is in the flat position so thew block will keep it up. It's kind of a redneck fix but it does work and doesn't impede the usage of the seat (i have kids and needed the 3rd row frequently). Plus, it's pretty easy to grab out if you need to lay the seats flat to haul something..seatbelts still work, and nothing was compromised. Do NOT cut the motor loose and use the seat without means to hold it up. Might well be a safety concern that way.
With a 4x4 block there's no way the seat can lay flat until you pull it out. It's a snug enough fit and if you cut it to just the right length, it will clear the plastic trim that lays across the floor in that area. I left mine laying there but not fastened down so in order to facilitate removing the block as needed.