Diagnosis
Yeah, this one's common. The firebox door hinge pins are often installed too tightly at the factory, binding the door before the latch even engages.
How to Fix It
Grab a hammer and a punch or a thick nail. You'll see the ends of the hinge pins on the outside of the firebox. Give one a few solid taps to drive it out just a millimeter or two—don't knock it all the way out. Do the same to the other pin. This loosens the hinge's grip. Open and close the door a bunch of times to work it in.
If That Didn't Work
If it's still stiff, the issue might be the latch hook bending inward and grabbing too hard. With the door open, use pliers to gently bend the hook on the door outward, away from the firebox, just a tiny bit.
Heads Up
Wear gloves when you do this—those edges are sharp. And go easy on the bending; metal fatigue is real. A little adjustment goes a long way.