Having to re-enter security code?

I have a Fiesta i.4 Style TDC1 with Sony 6000CD installed. Only problem every time I get into the car I have to re-enter the security code. I go through normal procedure using buttons 1-4 for setting the 4 digit code and button 5 to enter Audio now plays I finish my journey switch car of come back a couple of hours later and have to repeat the process again. Am I missing something no info for this prob in manual.

Sam from Nottingham
January 2012