Birds (Mallards too) will turn their eggs when they have gone broody. They also get jostled around when the bird goes on the nest to lay a new egg as well. Sometimes you can tell a 'newer' egg from an older one by the looks. Older eggs can look 'shinier', probably due to the oil that is picked up from the hen. Newer eggs look less shiny. If in doubt, choose the top eggs, as they are more likely to be fresher.
Hope this helps!

Thank you so much for your help. I will try that. I took them all and I will incubat them for 10 days and candle them. Maybe this will help also.

Again, thank you

Melissa