I have a handful of unfertilized eggs sitting on the ground in an animal zone that contains hens and roosters that are all adult and healthy. They have been there for at least 2 days and the eggs have yet to get fertilized. Why haven't they gotten fertilized yet? I seem to remember this not taking so long previously.
Ultimately it's RNG. Adult hens will lay fertilized eggs (white eggs, not tan colored) during spring and early summer providing there are males present. They don't become fertile while on the ground. If you see a lot of vomit on the ground in the animal zone they are in, you might try switching out their diet some to or from hay with other vegetables for a while and see if that helps. If you see white eggs on the ground, be sure and check them as forbidden to colonists to prevent storage or use for cooking.