It's happened to me a handful of times. My trailers use air to pull the pins inward, allowing it to slide. If the pins dont go in, you can try taping it with a hammer to jerk some of the gunk loose. I wouldn't smash it with a hammer, just lighter more consistent tapping is probably all you need.
Rock the truck back and forth, then try it again. More tapping. Make sure the ground is level and your truck is straight.
In my case they have all eventually given in and I was able to slide them. But occasionally it can take persistent work and patience. lol