Footprints are most likely to be created between cycles of deposition. Periods without deposition, often known as hiatuses , can represent hours, days or even years. The preservation of footprints relies on burial. Exposed prints will be subject to forces of erosion, especially wind and rain. Therefore we are more likely to find ichnites in rocks laid down by reasonably regular cycles of deposition.
Natural casts tend to be more durable than natural moulds.
Moulds can be easily overlooked since they often erode out from underneath sandstone ledges. Casts can be easily missed as well. Sunrise and sunset are the best times to look, when long shadows make subtle differences on rock surfaces more obvious. Cretaceous limestone moulds filled with clay. Dinosaur Tracks and other fossil footprints of the Western United States.