Making pickled grapes. After processing in the hot-water bath, they leaked brine. I later heard a couple of "clicks," but I'm going to presume they didn't seal. My guess about what happened: I didn't leave enough head space.
Here's the question: will they explode? Should I just unseal them?
If you didn't get a good seal you can do one of two things. Put it in the fridge and begin using it up keeping it refrigerated between uses or, if it you make the discovery and act promptly, reheating the pickles and repeating the canning process, ensuring you get a good seal the second time.
A good seal is indicated by a depression in the lid and the ability to pick the jar up by only the edge of the seal. If the vacuum holds the weight of the jar and food you know it's secure.
Your guess about the headspace could be a good one. Headspace is necessary for the air which expands during the water bath to evacuate for a vacuum to be created. But other things that are possibilities are a flaw in the soft sealing material on the inside of the lid or a chip in the edge of the jar. Any port that permits any exchange of air is enough to interfer with the process. It's a good idea to run your finger around each jar and lid before you boil them.