There are three levers to control the middle row seats:
top of the seat - used to climb into third row
side of seat - reclines or folds seat flat
underneath front of seat cushion - moves seat forward and backward
You need to use the third lever, which is a steel bar underneath the front of the seat cushion just above the floor. You may have a hard time reaching it because of how far the seat is moved forward. Or the middle seat may be wedged in place by the front seat. Try moving the front seat forward to give you more room and to release pressure on the middle row seat, then reach underneath the middle row seat cushion, lift up on the bar and slide the middle seat rearward.
If that doesn't work, I would try releasing the top lever simultaneously with the lever under the seat because something may be jammed.
In that position, lift the top handle again. The seat will lift up. Use the bar underneath to slide the seat back in the lifted position. After sliding back, lower the seat back to its normal position and it will fall back with a click