Another thing that might cause all leaves with no bulb is crowding. They won't form bulbs if they're too close together. Give each plant 1 1/2 to 2 inches of space.
Too much shade and too much fertilizer can also cause only leaves to form. Give them a sunny place and don't fertilize them. They're a crop that truly thrives with negligence. I just toss the seed pods all over in the fall, then they come up like weeds all over the garden in very early spring. I pull them when they're ready to eat and if I have too many, I dig them under to nourish the soil. You could try planting a row in early winter, like between Thanksgiving and Christmas, right before the ground freezes, and then just wait for them to grow in spring. You might want to buy the seeds now while the stores still have them. Or, let your all-leaf plants just stand there. They'll grow tall over the summer (maybe 3 ft or taller) and eventually flower, then form little green pods that look like miniature pea pods. Those pods, if picked small, are edible and delicious with a little bit of zing, and I've seen them in farmers' markets. Goldfinches love to eat them, too. Let some of the pods mature fully and turn brown. When they are brown and dry, you can collect the seeds from the pods and plant them or just toss the whole pods around like I do.
Mmmm yes, those home grown tomatoes are so good.