I agree with much of what has been said. As a real estate attorney my office handles a number of short sales, and there are just so many angles, that you have to take it one step at a time. However, what I have started to see lately (particularly on higher priced homes $750,000 and up) are that the appraisal (or BPO) of the home is coming back for more than the Buyers are willing to pay. So part of the process of the bank accepting a short sale offer is that they send out an appraiser to assess the value of the property - or what it should sell for (sometimes they send out a third-party realtor who does a BPO or Broker Price Opinion). If this BPO or appraisal is higher than what the purchase price of the property current is at, then the bank will generally not move forward. Banks what the purchase price to be at or near market value.
That being said, the other point I would like to caution everyone on (at least in Illinois) is the use of non-attorney negotiators for short sales. I have spent a decent amount of time cleaning up messes left by third-party programs who move forward and get a short sale approval for a seller, then they come to me to complete the transactio (yes yes - everyone out there from other states - we here in Illinois, in the Chicago-land area use attorneys on residential buys and sells). In many of these instances before going to the bank these so-called negotiators do not check title to the property, and therefore do not know of other liens that might effect the bottom line of the proeprty and therefore the amount that they can get to the bank for the short sale. Real estate tax liens, other mortgages, judgments from divorces, etc. The attorneys in this state that do short sales negotiations check these things BEFORE going to the bank for a number. I know it is a round-about answer, but if the bank thinks they are getting one number, then there turns out to be another lien on the property, the deal will fall apart then as well.
There are many real estate attorneys in Illinois that do short sale negotiations, and they charge the same way a neogitaitor should - they only get paid if the deal goes through and the fees come out of the proceeds of the sale, so that the seller does not have to come out of pocket for that expense.
Hope that helps,