New and old batteries both refuse to charge

I purchased a new battery from Ifixit for my Galaxy Watch 4 Classic. I followed the installation guide very carefully (三星 Galaxy Watch4 Classic 电池更换) and had no issues.

At first it worked fine: I charged it up, ran it down completely as instructed, and then went to charge it. The watch would charge for a few seconds or minutes, maybe showing 0% to 3%, then stop. If I tried again (and again) I'd get the same results.

Finally I gave up, assuming the battery was faulty, and put the old battery back in. It had about 14% power but immediately ran down to nothing and the watch shut down. It's refusing to charge again.

What am I missing? Could there be a bad connection to the battery somehow? Damage to some other part of the watch?