The return air filter. Most systems draw all their air back through one or two grilles, usually in a hallway ceiling. A clogged filter starves the fan coil, so cooling weakens and the coil can ice over. The fan coil in the roof. The indoor coil, blower and drain tray live in the roof space, out of sight, and a Melbourne roof space on a summer afternoon is a harsh place for them to work. The zone dampers. Small motors open and close each zone; when one sticks, a room goes cold, hot or dead. The ducts. Flexible ducts get crushed, split or knocked off their outlets. The outdoor unit and controller. A blocked condenser, a failed fan or a lost signal between controller and unit stops the lot.
Our guide on why a ducted reverse cycle is not cooling covers the symptoms one by one.