According the National Ground Water Association:
Along with the water table dropping… the most common reason for water production drops are the plugging of holes along the well’s casing and encrustation forming on the well screens. The amount of water going through the well system will drop significantly if several holes or portions of the screens are clogged. Calcium carbonate, iron bacteria, manganese bacteria, silt, clay, and ‘slime,’ a combination of sediment and deposits, are all common well clogging issues.