2018 Kenworth T680 fifth wheel won't release kingpin

Lift the release handle and move it slightly forward

 

I had exactly the same issue and new to me 2 weeks ago. I just did the opposite of everything I've ever done in the past in a different truck. I set my tractor brake, not my trailer, jump out pull the pin, then set trailer brakes and continue the drop process with landing gear & lights.
But ABSOLUTELY TRUE. No pressure on those jaws or they won't open. I still hold my breathe when i pull that hook. At least for now.

 

Typically this happens at midnight after driving 11 hours with a hot shower and spaghetti waiting at home. I spent half an hour with it the other day. Tried reversing against the trailer, dropping the air, etc. This is a new truck for me but I found with my old truck if I didn't set the trailer brakes and pull the pin before dropping the landing gear it works bind. In any event, eventually I applied the trailer brakes and pulled on it. Then set the parking brakes on truck and trailer. Then I lowered the landing gear and then I dropped the tractor air. When I did that I heard what sounded like a small metal plate dropping into another piece of metal. Then it released effortlessly.