My Philips R760 Car Radio doesn't accept my valid code?

I have a Philips R760 car radio, which doesn't accept a valid security code. The radio is supposed to display "wait" for each invalid entry, with the time doubling every time. The "wait" does get displayed everytime, but only for about 1 second. Can anyone explain what might be causing this and how to correct the problem, so my code is accepted, please?