Jars don't always seal immediately as soon as you remove them from the canner. In fact they seldom do as some cooling is necessary. And if the contents is still actively bubbling it can take a hour or two for them to seal.

That's why the directions say, "Let the jars sit undisturbed while they cool, from 12 to 24 hours. Do not tighten ring bands on the lids or push down on the center of the flat metal lid until the jar is completely cooled." Then you check the seal.

I always check the seals at about 10-12 hours as that gives me time to reprocess them if necessary.

Freeze the jar in question since you can't know now when it sealed and it has been more than 24 hours since it was processed.

Dave

Here is a link that might be useful: Step-by-Step Guide to Using a Pressure Canner