I grow brussel sprouts every year. I get very large, seemingly healthy plants, but very little sprouts.
I grow in raised beds. I have made sure soil in compact. I get very good production from Kale & Broccoli. Soil has plenty of compost thus I do not add a lot of fertilizer.
Water and feed Brussels sprouts throughout the growing season. Keep Brussels sprouts evenly moist; do not allow the soil to dry out. Side dress Brussels sprouts with bloodmeal when sprouts begin to form. For a large number of sprouts let plants grow tall and continue to produce sprouts. For full, fat sprouts, pinch out the growing point to stop upward leaf growth and allow sprouts to form on the bottom 12 inches of the stem.