The First Colonists landed on Crèche some sixty years ago, the vanguard of what was intended to be a grand colonizing caravan that would bring hundreds of thousands of other nouveau Crechi and Crechae to this toasty-warm lava-ball. For Reasons That Have Never Been Adequately Explained (i.e. a computer error that the damned sneaky machines managed to cover up before human auditors arrived), all of the subsequent colony ships were diverted to the beautiful planet of Mootikki, famed for its semi-sentient water spiders that eat anything with a pulse. Forty years passed before some minor functionary stumbled across records of the original Crèche expedition and sent out a scout to peek in on their progress.
Crèche had not done badly, all things considered. There had only been ninety people on the lead ship, but there had been a full complement of builder-bots, plus all the materials needed to erect a life support dome and get the food vats pulsating. In fact, with only ninety people to support, there was an embarrassment of supplies, and more than enough bot-power to keep the staples stapled.
All skittles and beer then... except for The Problem. As reported by the Crechians to the scout four decades later, no children had come. Hell yes, they had tried making babies, with a lusty devotion, and according to all medical analyses, they were fertile as Teenagers-Who-Think-It-Can't-Happen-To-Them; but something in the water/air/top-soil/Van Allen Belts was preventing Mr. Sperm and Ms. Ovum from producing nicens little baby Zygote, and year after year went by without those little feet a-patterin'.