I’ve rebuilt the swivels, they are pretty simple once you get them apart. I recommend getting new stainless hardware from McMaster Carr. The reason for the swivel not returning is the spring and ball bearing assembly. A lot of mine were rusty and gross.
Usually it takes a new spring. The originals will have bent or broken, letting the ball bearing that rides on top of the spring to fall out, or otherwise jam up. The ball bearing rides in a little plastic cup on top of the spring. That can fall apart too. The cup was the hardest part to replace for me. I found little metal cups at the hardware store that fit in the top of the new springs I bought. The cups I used are axle push nuts. They fit on the end of an axle rod to hold on a wheel for something like a hand truck or dolly.
It’s really satisfying making them brand new and smooth swiveling again.
McMaster Carr has the ball bearing thrust washers that go above and below the swivel shaft. They also have the weld nuts. Pretty much everything needed except the push nuts. They may have those too, I looked but couldn’t find them.