According to recent research, and this is still true in 2013, the delays may be caused by:
(a) buyers not being qualified to buy the home in the first place;
(b) lenders over-promising and not able to deliver;
(c) appraisals that come in lower than contract price; and
(d) both the buyer and Seller can’t reach terms on the contract price.
Source: http://www.latimes.com/business/realestate/la-fi-harney-20120401,0,6931944.story