Why were my brussels sprouts small last year?
The plants looked reasonably healthy - not much leaf damage and about 2 feet tall. Because it was my first year gardening I planted the sprouts (and everything else) in late May and spread an inch of grass clippings soon after. I planted them on the west side of a fence and they got 6-8 hours sunlight daily.
Of all the factors in growing conditions and maintenance that contribute to plant health and productivity, soil and water management are most important.
You mention mulch with grass clippings, but if that all you did to improve soil fertility then maybe its not enough to acheive the nutrient availability that the plants need to flower and produce.
Next season incorporate some good quality compost into the soil before planting. Fertilize with a complete organic fertilizer like this:
Grass clippings alone, as mulch on the soil surface often form a matted layer that does not allow good water infiltration. Best to mix with a courser material, or compost the grass and then use the compost as mulch.