My guess is you are seeking a foreclosed property.
Many people chasing distressed property sales(foreclosures) are meeting with this scenario. The fact of the matter is, many of these properties go to contract in a matter of a few days and the listing agents are being bombarded with calls and requests for information.
Agents that are persuing these properties in support of their buyers are meeting with this frustration as well. It insensitive to only be able to reach an answer machine, leave messages, and never get a return call.
Generally, most professional agents return calls and seek to establish relationships with potential buyers.
Keep trying. The right opportunity is out there.