Seems this has affected a few people, so I'll let you know how we solved the problem.
Issue? While breeding rexes, poor Jesus realised that the fertilized egg had dropped through the floor of the map. We both lay down, pressed K, and sure enough, there it was, out of reach. The solution? Egg and Poop collector

If you can, use the demo gun to quickly pick up the nearest egg and poop collector (check it's in range of power where you're going to place it) and place it down roughly above where the egg is. Switch it on, and the fert egg should appear in the collector. Worked for us, and can be done quick enough to return the egg and poop collector back to its original location.
Good luck.