I have a 56k modem but am not getting a 56k connection.

Your modem may be capable of achieving 56k connections, but the phone lines are not.  FCC limits your phone line to a maximum of 53.3k under perfect conditions.  Even then, you probably will not get 53.3k.  Your connection speed varies almost every time you connect according to line conditions, weather, phone routing, etc.  There are lots of things you can do to ensure your connection is the fastest possible.

If you have tried all the above solutions (some of them may not be possible according to your situation or experience) and still have no luck, you can follow this link for some more options. 

Link to other information  U.S. Robotics Sportster 28.8 Modem Loses Connection Speed

Problems connecting at High Speeds with US Robotics 56k WinModem, LT Winmodem or HSP Winmodem.