It's broken? If you have it off you can see that there's not much to it but if it's physically broken, I doubt there'd be a good permanent fix unless you can creatively "rig" something.
It moves very slightly so if it's not really broken, then perhaps it's just not moving correctly when screwed into position? Are any parts of it binding or catching on something when in place? Was some carpet removed, cut out under the area where the release lever is because cutting that out is part of the instructions and if not done, the lever will bind and not release well. If it seems to be binding, silicone spray often helps. The complete assembly is also adjustable side to side as you can see w/o the upper seat back installed so you can temporarily install that section w/o the seat back and experiment with adjustments until you get it right if need be. I usually don't have too many problems with those but on occasion one might need more attention than others.
Hope this helps?