How do you unstick the corner of a metal door?

Open the steel door. Hold the front edge of the door and try to move the entire door up and down. If the door moves, the bottom screws on the hinges are loose. Hold the front edge of the door and try to move the entire door back and forth at the hinges. If movement is possible, the center screws on the hinges require tightening.

Tighten the hinge screws. Close the door for proper fit. Check the door for movement. If the door still moves when you hold the front edge and move it up and down or side-to-side, unscrew the screw causing the movement. Insert the tip of a toothpick in the screw hole. Place the screw in the hole and re-tighten. This particular step indicates that the screw holes have become larger, and the toothpick fills in the excess area.

Look at the front of the door and check for spacing between the front edge of the door the door frame. The spacing should be the same from top to bottom. Determine whether the top or bottom hinge is causing the spacing problem. The hinge at whichever section of the door is farthest rom the frame is the problem. If the top is farther away from the door frame than the bottom edge, work with the top hinge for spacer placement.

Remove the top and bottom screws from the hinge. Place the screws in a safe location so they do not get lost. Unscrew the center hinge 1 1/4 turns.

Slide a plastic door shim behind the hinge. The center of the door shim has a slit cut from the outer edge to the center of the shim. Slide the center screw into the slit. Continue to slide the door shim behind the hinge.

I have even used stiff cardboard behind hinges as shims.

Replace the loose screws. Tighten all screws.

William's answer was perfect. I would also wax the doorframe to make the opening and closing smoother.

Use a level to check for plumb then adjust the screws accordingly. There's a wonderful product called Aluminum Lubricant & liquid masking. It's a spray for windows, sliding doors, awnings, hinges, etc. I personally have used this on my sliding doors, windows, and even ladders. I can open my old glass slider with two fingers! Best of luck.