5th wheel hitch problem... Will not release

Front landing gear on ground, chock rear wheels, I lift fifthwheel so that I can see the pinbox is off the hitch maybe 1/4". I get in truck, drop gear shift into reverse, let off brake and just barely move truck back to take kingpin pressure off truck hitch. Get back out, try to pull handle.

If you have done all of this, maybe try to get in truckbed with some lubricant, I use silicone spray mostly, and spray what you can reach. BE CAREFUL. PLEASE.

Sometimes, I might have to stand on a stool, to get a little more leverage and strength in pulling the handle.

Try Chalking the trailer wheels and throw the truck in reverse. Push the truck backwards into the hitch very slightly to take the pressure off of the bar/jaws. Hopefully that's the problem...

Sounds like it is bound up. Are you trying to unhitch while on an incline?

You need to tell us what hitch you have. In general, you should lift the weight off of the hitch before pulling pins.