Ask your self and your Realtor, "Did you offer a good price"? In my experience as a Realtor working Short Sales for my clients, whether buyers or sellers, the bottom line for the bank is MONEY!
In my experience the 1st reason a buyer doesn't receive a response is that the bank isn't going "to waste their time" with an offer lower than they want.
The second reason, I've experienced is that they just aren't ready to sell the home, possibly they don't have the deed cleared. Maybe it isn't in line for sale.
Back to the #1 reason, if the listing agent and the homeowners have gotten all their ducks in a row and have been fully approved for a short sale. (Thoroughly question the listing agent) Then question your offer price and how strong is your offer? do you have many contingencies? You need to look like a viable, strong buyer to the bank.
ps If you don't want to wait indefinitely, give the bank a deadline and wording that will allow you to withdraw your offer.
Good Luck--however it's more the skill and experience needed here!
Full time Realtor
"Your Short Sale Solution"
Realty Home Pride