Mitsubishi Lancer 2004 Radio Code Not Working

I bought my mitsubishi Lancer second hand. When I had the battery replaced the CdD Player and radio no longer worked for a period longer than five minutes. I approached the Mitsubishi car dealership for the code and they have given it to me. I have carefully followed the directions in the manual and I have the correct code but when I press the play button to enter the code, the code just winds back to 4 zeros and doesn't work.
I had the same problem on a previous occasion involving turning the key without starting the ignition and entering the code but I can't remember exactly what I did to make the radio work...Has anyone else had this problem?