If you're pressing down and the lid is not bouncing back, you've got a good seal. You may have to wait to the jars to cool completely after cooking to have the vacuum action work as well, so be patient!
To the first part of your question, if when you can your jam a jar doesn't seal take the jam out of the jar and put it in a clean, sterilized jar, with a clean lip. Also put on a new lid under the ring. Check the old jar and see if it has a chip in the lip. Or perhaps it didn't seal because the lip was dirty.
SKK is right. The usual reason for a non-seal is a bit of jam on the lip of the jar. A canning funnel helps prevent this, along with careful wiping of the rim and threads with a damp towel before screwing on the lid. And, tempting as it may be, never press on the lid; let it seal by itself. You can always refrigerate a jat that doesn't seal, or even freeze the jam.
HalfPint is a trusted home cook.
For your first question...No, you can't just re-boil it. If the lid does not seal, check the jar lip for cracks. If all is well with the jar, wipe down the lip of the jar, get a new lid and re-process the jar.
The second question: yes, when the lid stays pressed. It's sealed, though when it doubt, leave that particular jar in the fridge and use it up first.
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