I've had a garlic tragedy, and I'm looking for help on what went wrong. This is my second season growing garlic. The first season, I put in very little effort, made a bunch of mistakes, and still got a lot of good bulbs. But this season, using some of the exact same garlic as seed, and trying to do everything right, I expected much better results; instead I got consistently tiny garlic heads!

Background:

• Sunnyvale, CA - Zone 9b
• Backyard in suburbs

The first season, that went well, despite half-assed approach:

• Spring planted Feb 10
• 56 cloves of "Gilroy garlic" (presumably California White) from a local grocery store (and also an unknown, probably Chinese garlic that only ended up yielding "rounds")
• Harvest June 19
• Yielded 6 large cloves, 24 medium-small, and 7 too small, and 4 injured
• Location: Ground in front of back fence of residential back yard, full sun
• Soil: Hard clay dirt, top-dressed with some gross stuff from my gutters, added all-purpose slow release fertilizer
• Spacing: sort of random, maybe 4" in 12" rows (with a lot of encroachment by other landscaping)

The second season, that went poorly despite loving care:

• "Vernalized" in fridge Nov 5-Nov 29
• Planted fall Nov 27
• Varieties: Replanted Gilroy and unknown above, plus Music and Red Chesnok from "Green Mountain Garlic" in VT
• Harvest June 19
• Yielded a bunch of cloves about half the size I expected, very consistently across all 4 different kinds, including the largest heads/cloves that had been successful a few feet away! The Music came out the largest, but again, about half size from what I was hoping for. All of the heads were all a very consistent size, and divided into cloves, just way too small.
• Location: 4x8 3' raised beds a few feet away from the old location, full sun -- that I hoped would be less shady because it was higher up
• Soil: Commercial bulk raised bed mix, with some bagged soil mix and compost mixed in, and a small amount of local dirt
• Spacing: 6", 5", 4", 3" (I did an experiment, see below)
• There were no obvious signs of disease growing or at harvesting, except more rotting in the wrappers than there should have been.

Possible causes I've considered, in the order I think they might be related:

• Watering - When harvesting, I noticed it was really pretty wet in the raised bed about 5" down. (I also forgot to stop irrigating a couple weeks before harvest.) The roots looked OK, pretty long and full, but a lot of the wrappers looked in worse condition, more rotted, than I think they should have been. So maybe it was too wet? But the first 1-3" or so were actually fairly dry.
• Not enough mulch - Sort of related; I had a really thin 1/2" layer of chopped leaves as mulch. I could probably have more, and maybe that would make the moisture gradient more consistent.
• Vernalization - I sort of forgot about this and ended up only vernalizing about 3.5 weeks in my fridge. I didn't vernalize at all the previous planting; but maybe the supermarket garlic had been in cold storage? Also, I thought Gilroy garlic probably shouldn't require it.
• Temperature - I checked with a kitchen thermometer, and the raised beds seemed a very similar temperature to the soil I had planted the previous season. But it's possible it was a few degrees warmer on average, or warmed up faster, or something, and that was enough to impact the growing.
• Spacing - Maybe even my 6" equidistant spacing was too close.. although I have reason to doubt that below
• Fertilizer - I used slow-release nitrogen fertilizer pretty early on, and I noticed at harvesting that some of the granules were still intact. I wonder if I could have used too much? Also, there was tons of organic material, also possibly too much.
• Soil contamination - My soil mix included 3 foreign sources, including municipal compost. That makes me really nervous, that some constituent might have had either a chemical herbicide, or some other contamination that could inhibit growth. I've also concurrently grown other things in the bed, with some successes and some failures, but I'm not really sure.
• Sunlight - There's a lot of things that block sunlight at various times a day, including the row covers I had to make for the raised beds. I still feel like it's full sun, but perhaps the blockage is worse than I thought, especially in early spring months?

For the second season, I was hoping to do a little study on spacing. I planted an equal amount of the 4 varieties of garlic at 4 different spacings. I was hoping to draw some conclusions on how closely I could space plants without impacting bulb size.

Here is the result:

Spacing in in: 3 4 5 6

Gilroy 10 13 21 20

Unknown groc 14 14 18

Music - 46 56 55

Chesnok Red 21 26 36 27

(Average weight of garlic heads in grams, by inch spacing)

Because 5" and 6" spacing yielded very similar bulb sizes, I concluded that spacing was probably not the issue, and that going up to 7" or 8" wouldn't change anything. I could be drawing that wrong conclusion though.

I know this is a ton of information. Is there anything that jumps out that I might have done wrong?