Why is my failure to seal rate so high

Hello, I'm quite new to canning, basically only having done it for a couple of months to pickle the cucumbers I'm getting from my garden. No matter what, I seem to have at least 1 jar not seal out of a batch of 6 quart sized jars. I feel like I'm doing everything exactly how I am supposed to, so I wrote out my process so hopefully someone could identify something I'm doing incorrectly.

I'm using the raw pack variant of this recipe, although scaled down a bit. I live in the 0-1000ft elevation zone.

1. Thoroughly wash jars and rings (always handwash the rings but sometimes I use the dishwasher for jars), and ensure that they're dry

2. Fill jars with sliced cucumbers, while agitating and packing them, making sure I stay at or below 1/2" headspace

3. Fill jars with homemade brine from recipe (hot) to 1/2" headspace

4. Use a wooden skewer to remove as many air bubbles as I can, then top the jars off if the liquid level drops any

5. Using a damp cloth, wipe the entire top of the jar, then wipe with a dry cloth. I very carefully inspect each jar to make sure there is absolutely nothing on the sealing surface. I don't feel or see any nicks or chips.

6. Place a brand new lid from the box on top of the jar (I've tried both Ball lids and Mainstays lids; both have had the same failures to seal), and with the jar on top of a towel on the counter, tighten until jar spins. I then, using only 3 fingers, snug up the lids a bit; definitely no more than 1/8th of a turn. Very conscious of not overtightening here. Also, I always very carefully inspect the actual sealing compound on the lid to make sure there is nothing that would hinder a seal.

7. Using a jar lifter, I drop the jars straight down (no tilting) into a pot which has the thingy on the bottom so the jars don't directly touch the pot. The pot is full of water which is close to boiling. Enough to cover the jars with an inch of water.
   Note: I feel like I am at the very least not overtightening my jars, because when I drop them in, I see air escaping from all of the jars.

8. Use high heat until the water begins to boil, start my 15 minute timer, and then lower the temperature so I just maintain a mediumish rolling boil – not enough to make a mess, but a slightly aggressive boil. I also have a lid on the pan at this point.

9. When timer goes off, I remove the pot from the heat and remove the lid, and allow it to cool for 5 to 10 minutes before doing anything else.

10. I then, using jar lifting tongs, lift the jars straight up out of the water and place them onto a thick towel on my counter. I don't touch them at all after this.

After very closely following the above steps, I still will have a jar that doesn't seal up after 12 hours.

This morning I carefully inspected the one out of 6 that didn't seal, and there was a clear indent on the lid where it was against the jar, EXCEPT in one little spot where there was no indent in the sealing compound. There was nothing but water in this location; no food residue or anything. Interestingly I did notice that where the threads on the jar are directly below the spot that didn't seal, there was some seasoning that definitely wasn't there when I dropped the jars in. But there's nothing anywhere near where the lids actually seal. Maybe the contents of the jar are boiling out and leaving invisible residue? This pickle brine is very sugary. Am I just not tightening them enough?

I'd be very thankful if someone could help me out, because this is becoming very frustrating. Sorry for the long post!