My biggest mistake with growing garlic was in buying without paying attention to suitability of the variety to my region. I accidentally (didn't know there was a difference) chose a cold region variety, but am gardening in warm South Carolina. The garlic bulbs just divided in the spring, and only gave small bulblets rather than full bulbs with decent sized cloves.
My second biggest mistake was planting in the spring, as especially in the south, the bulbs need the added growing time that is provided from fall through winter. Plus fall planted garlic is well established by the spring and is ready for rapid growth in the early spring. That period of rapid growth is necessary for good bulb formation. A spring planting handicaps garlic to the extent that bulb formation will be weak and results will be mixed between small to medium single bulbs or very small bulbs with almost unusably small cloves.
My third biggest mistake was in planting too early in the fall, while the weather was cool enough to allow spouting but was still very warm for a couple of months. Those plants tend to divide, giving clusters of garlic in the spring. None of the bulbs get very large, but they are still usable. The results are very similar as to when planting a cold weather variety in a mild winter area.
My best success comes with choosing varieties which are supposed to do well in my climate and when planting them at the proper planting time for here which is November, near the time of our first frost. Those plants usually grow well through the winter. They usually remain as single stalks which make large mature bulbs with very nice sized cloves.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.