Sometimes they 'bury' the eggs so they are not easily seen. When I was a kid there was a mallard hen that had a nest in the junipers. A raccoon discovered the nest and destroyed the eggs...however as my dad was cleaning up the mess, he found two that were 'buried' in the nesting material, I actually got them to hatch. It's possible that the mallard just isn't laying an egg every day. Or maybe she's not putting all her eggs in one basket and has two nests, ha!