Certified GKH Gardening Expert
It could be the wrong type of fennel. For instance, Florence fennel is a bulbing type. Wild fennel is not. Did the seed packet specify it as a bulbing type? Also, did it flower? If so, it could be due to bolting. If allowed to bolt, the plant will put more energy into making more seeds rather than bulbs. The only other suggestion I can think of is earlier planting. These plants need cooler weather to produce bulbs, so perhaps you should try planting yours as early in the spring as possible, as soon as the ground can be worked, so that the bulb will develop in cooler temperatures.