So sorry but I think you can give up on those cosmos seed if you have not seen  seedlings after 6 to 8 weeks. It is most likely they sprouted but then were eaten by slugs or birds. Try covering the seed patch with netting after planting and using a slug bait to protect from slugs. Planting too deep or letting the soil dry out will also cause seeds to never sprout. Keep growing, Marianne Binetti