The very first thing I ever pressure canned was Chili. I had 2 jars out of 7 quarts seal. (I think it's still on the shelf.) I was devastated. Like cussing mad. I had a shit-ton of siphoning and a huge mess inside the canner. I had followed the directions in the pressure canner book. There's a nuance to everything.
One of my jars of Roasted Marinara came unsealed sometime and left a sticky mess on the shelf. I didn't find it until I reached for the jar and it was stuck down. Then my dumb ass lifted the lid..... retch. I hurried up and dumped the contents into the compost and got hot water and dish soap into the jar. Jar has now been cleaned 3 times and refilled; back on the shelf.
Shit happens. There are sooooo many teeny tiny steps that make a difference during the canning process. That means that sometimes things go wrong. Very often, the lack of a good seal is evident right after canning and you can save the product to eat right away. Other times, the seal breaks on the shelf and you have a mess on your hands. Clean it up and move on. Yes, there was effort, time and money inside that jar. It wasn't the whole batch. Try again another time. It's ok. You've wasted more time, effort and money on your ex than that one jar that unsealed.
The jar rim can have a nick in it. I might have reused a lid that couldn't handle the pressure. There might have been a speck of food on the rim of the jar. There might have been siphoning during canning that let some of the product out onto the rim. The sealant of the lid you used didn't like the vinegar you wiped the rims with. I might have accidentally touched the sealant on the lid and the oils from my hand were enough to mess up the seal. Todays flat lids are much thinner and don't have as much sealant on them as they had in days past. There's a multitude of things that can go awry.
Just keep trying. The more shit that goes down, means that's more that you've learned and can do differently.