Why amphibious landings usually prove difficult

Why were amphibious landings in Italy and elsewhere so hard, with smaller forces than planners wanted?

Available forces, infrastructure, and intelligence constrained the landings. Normandy benefited from unusually extensive preparation and information.

An amphibious landing depends on what forces can be launched, what infrastructure supports them, and how much information planners can gather. Normandy was relatively successful and straightforward in this comparison, with an unusually large amount of information available beforehand. Treating it as the normal standard obscures the constraints faced by other operations, including those in Italy. Island campaigns raise a separate geographical consideration because troops can potentially land around much of an island's coast.