Tandems won't slide even with pin pushed in

It's not orthodox... but, make sure your brakes are adjusted, the pins are pulled WELL CLEAR OF ALL FOUR HOLES..
start rolling backwards at about 3-5mph and pull down the trailer brake.
That should jerk it out of its bind.

Just be ready to stop when you feel it give.

Disclaimer
I did not.. I REPEAT.. I did not say to go wide open and slam the trailer brake down and loosen it that way.

 

This happened to me once. I ended up spraying a half a can of "liquid wrench" into the holes and in and around the frame and slide rails. Let sit for about five minutes and was good to go. You can purchase it at Walmart for a couple bucks. Very good stuff.

 

And to be clear. You can only do this ^^ when you have a “lever release” system.

 

Or you lock button from closing with visegrips and slide it like a lever lock system..

 

Use a chock behind front tires on trailer go slow until it slides

 

Heavy sigh...

The red knob controls a spring brake. Springs wear out.

Use the trailer hand brake instead. It's an air brake.

 

Tried that once. Ripped off the mud flaps...

 

And that's why I always, repeat, ALWAYS, check both sides.