In case anyone is interested in the future: I called Presto and they're sending me a new plug, no charge. They also explained that when I first insert the plug, I need to do it from the inside and push out. But if I remove it, I should reinsert it from from the top and push down. I won't be removing it, I'll just go back to my old water bath canner.
I can't use the pressure canner until I get the new plug so my broth will have to go in the freezer.