I didn't really want to contribute to this thread because when something doesn't grow well, I keep trying until I figure out what to do. So not growing at all changes to growing and producing but not as well as I want them to, to finally a harvest I can show off.
Sometimes its because you need to sow directly in the ground, or because you need to start earlier... Or later. Sometimes soil pH is a major factor.
For me the more troublesome ones are spinach/beets but I think I needed to add more lime than I thought I needed -- lettuce and carrots grows fine so I thought I was adjusting enough, but I suspect they need more. They are growing better this year.
SVB'S give me more grief. Summer squash I can get by by planting as early as I can and grabbing as much harvest as I can before they arrive, but winter squash and pumpkins have been a disaster. So I'm hoping C. moschata varieties will do better this year.
Heat-loving long season crops. I'm taking a break this year, but I'm still working on growing luffa.
And yes, Brussels sprouts is another one of my "projects". Timing to start/plant as well as pests (cabbage loopers/moths, cabbageworms/butterflies, diamondback moths, as well as harlequin bugs). Trying Falstaff red/purple Brussels sprouts this year in hopes of alleviating some of the pest pressure.
Growing corn in a small garden. I'm getting good harvests but some spotty pollination is still an issue. But I don't think I'll overcome this unless I'm willing to dedicate a sizable space to corn and I always have so many different things I want to grow....