Very old thread but in case anyone else has this issue, I would suggest that you check to see if you have your antenna connected to the Tx/Rx connector and not the Rx connector, assuming you are using a duplexer.
Do not use prolific cables. Use FTDI cables. Buy Two Radios sells what is called a XLT Painless Programming Cable. This cable is $17.99, it is ab FTDI and will work.