Straight answer home is not available. They may be trying to modify and keep their home these are distressed sellers if they were considering a short sale. Put yourself in their position and ask yourself what your reaction would be if a stranger knocked on your door demanding to know what the status was. If you are being told the property is not available then that is the end of it. They are steering you to other properties because they are being helpful. Get yourself an agent that can show you properties that are available.
If the house is truly listed as "active" in the Realtor MLS system (your Realtor can verify the current status), then there is another possibility of why you're getting the run around.... fraud.
I've seen many listing agents and sellers and 3rd party middleman investors (or option holders) who will engage in what I believe is fraudulent activity. The insider investor pays the seller a certain amount of cash to "go along" and the investor then submits a lowball offer with fraudulent, overpriced repair estimates. Some listing agents may be evasive and not reveal what is truly going on.
The FBI is cracking down on these scammers and also lenders are now requiring more information from the buyer on the contract to prevent fraudulent short sales.
Good luck and should the seller accept your offer on a short sale, do not think you will actually ever close. The probability of a short sale closing at the asking price is slim because most are listed at an unrealistically low asking price.
In most cases the owner owes more than the house is worth, in a short sale the bank reduces what they owe so they can sell. if the bank wont reduce it, knocking on their door will not help you as they probabily owe too much to sell. They are losing their home either way you look at it by being forced to sell or by foreclosure, most people in that situation will not accomodate someone knocking at their door.
Please read my blog for more information on these situations.
However, upon reflection, I don't see a major problem in knocking on the door. Recognize that the sellers may be sensitive, concerned, nervous, anxious, etc., about the short sale. So they may not be fully receptive to even talking to you. So be careful in how you handle it. But, really, the transaction would be between you and the seller, with agents acting to facilitate the transaction. And it appears that they may be hindering, rather than facilitating. So--with the caution I expressed above--you might as well drive over there and talk to the owners.
I expect some other agents here on Trulia will be appalled at my response. And note: I do think you should have an agent representing you in this transaction. Absolutely. But if you want an answer to what should be a straightforward question and you're not getting it, there's nothing wrong in getting the answer some other way.
Hope that helps.