Obewan wrote:
...I've tried pulling the truck forward and even tried backward, raising more, raising less,...
I fought this at first, until I figured it out. You want to backup your truck, to be certain there is no pressure pulling against the jaws. I know you said you tried backing up, but the problem may be that you backup, maybe set your parking brake, maybe not, but either way when you take your foot off the service brake your truck wants to roll forward. And then you've still got the issue. Worst case is when you backed uphill into your site. Truck wants to roll on you.
Try having your partner in the truck, with the truck in reverse, holding rearward pressure against the chocked trailer, while you pull the release lever. I also find it helps to take the weight off the hitch head, with just a tiny gap between the plates, but it should work either way. If you're by yourself and have to do that, you'll have to chock the trailer, chock the truck too, and leave the transmission in reverse while you pull the lever. Caution! And I don't know what to do if you've got a manual trans.