IMO it may be the old across the bed from wheel wheel to wheel well hitch. My first 5er in '94 came with the brand spanking new across the bed 12k Atwood.
As the pin came in contact with the 1st latch and it closed around the pin the sliding bar didn't slide home all the way do to binding and no proper lube.
I disassembled the jaws and latch system and de burred lubed all the parts. Put it all back together and it worked very smooth for eight years till we sold the trailer and hitch.
I would turn the hitch over and R&R why the second click doesn't work. I have no idea where a manual could be found or even if there was one. Non came with my then new '94 Atwood.
'03 Dodge 2500 Cummins HO 3.73 NV5600 Jacobs
'98 3500 DRW 454 4x4 4.10 crew cab
'97 Park Avanue RK 28' 2 slides