Disconnected, Torn, or Damaged Ducts
Most ducts are fairly delicate and easily damaged. Duct systems should be inspected for visible holes and feel for air leakage while it is operating. Some duct sections that are supposed to be joined together may have pulled away from each other, leaving gaps through which large quantities of air can leak.
Flexible duct sections may have been torn during installation or afterward by someone carelessly crawling on them. Almost any duct can be damaged if someone steps on it or handles it roughly while moving around it.
In one attic, we saw a section that had been cut with a knife to leave a flap for cool air to escape. Apparently someone working in the attic was hot and decided to make himself or herself more comfortable by creating the hole. Unfortunately, the homeowner then pays the price with higher utility bills as their expensive conditioned air is dumped into the attic. It pays to check the duct work from time-to-time, or have a contractor do it for you.
Fiberglass ductboard sections are subject to damage if weight is placed on them. Whatever the cause, fixing holes in ductwork should be a high priority on any energy improvement list.