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Welcome, I'm Chris (aka Moose) I prefer to work when others don't, I am here around lunch and after dinner till after midnight.
Here are some diagrams.
Normally I find this to be a cable sticking and not retracting causing the hinge lock to not activate and allow the seat back to lock in place. I have a few other diagrams if needed. Can you check it over and possibly disassemble the seat to view things better.
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Inside the hinge there is a cam style locking lobe that engages and prevents the seat back from moving once engaged.
Since you have verified it is not a cable or linkage to blame and its not even locking into place while disassembled. That means its is a internal part in the seat frame hinge assembly https://dl.dropboxusercontent.com/u/90220402/seatframe.png