Lift lower base first. Handle is under side of cushion. Pull handle on top of smaller backrest and it should fold. Larger backrest second cos of the luggage net spool. If the backrest wont budge try pulling on the small knob beside the striker pin on the body. It has to be out for the interlock to work. If you want the luggage cover off it slides sideways with a dunt when the seatback is half folded.
Additional thought to what Will said. When the base is lifted there are little black paddles under the very bottom edge of the backrest. Make sure they are lifted up. They should spring up themselves but sometimes don't or jamb with something.
OK. We have to presume the smaller seat has a broken mechanism and this is holding the larger seat from moving cos of the load cover cassette. Unlikely that both catches are duff at the same time. iirc (it's been a while) the smaller seat is on the O/S. Pull off the edge trim for a couple of feet either side of the striker plate. This will reveal the three (?) clips holding the carpet covered moulding to the upstand. A screwdriver under them and ease them up. Through the gap you will see the seat mechanism.
This is a pic with the trim removed but with the backrest in place.
And this shows the two rods which operate the interlocks, the position they are in with the lock open and the seat folded, and the spring loaded over-center catch which flips open when the seat is pulled forward.
The rest is up to you to see what is wrong.
Hopefully, with the small seat working the larger will follow suit. Correction in post #2 re "Pull handle". Getting confused by W211 arrangement. Sorry.