Locking Jaw Refusing To Release Kingpin

You must be on a incline? Trying dropping the air bags. Then back against it like before

 

Lower the landing gear, (obvious, but I had to say it) and try (with your trailer brake set) rocking the tractor back and forth. If that doesn't work, and you already have your airbags deflated, you have a broken locking pin on the fifth wheel. I've had one. Good luck, even dealerships usually don't have those parts in stock.

 

Looks like the jaw adjustment bolt to tighten or loosen the locking jaw don't go messing with turning it unless you know what you're doing. To loose and you can lose a trl going down the road

 

That bolt has a spring behind it. Should be able to press it in a 1/4-1/2 inch when hooked to trailer. If no play in it that's your problem.

 

A little late I know, but...
If you are at a location with someone else around, ask them to pull the release as you back up. Set the trailer brake (or disconnect lines) release tractor breaks and apply reverse power until the wheels start to spin.
You can also try moving the trailer to another spot. You may be in a small pothole causing the trailer to not be square to frame.