When released, check to see if it's turning as it should when moving the release lever. If it doesn't move, you can remove the upper cushion (pulls off from the bottom clips and then slides upwards when tapped) and remove the catch/release assembly to inspect it and/or replace the assembly too as it's available as a spare part from dealers.
Check to see if material is binding under the release lever as it's suppose to have some cut out for easy operation when installing the 3rd seat. Not everyone installs those properly.
If the release lever moves properly when checked (released), it might then just need to have it's cable (cord) adjusted. It can be done with the seat back in place but it's easier if the seat back's removed. The end plate where it attaches is slotted and is adjustable by loosening the 2 screws and moving the plate. The release lever assembly is also adjustable, side to side with the upper seat back removed and all adjustments are just trial and error until it operates correctly. It can be adjusted to function perfectly however if there's nothing binding or broken.