Cant change the code on my veritas 8 panel!!?

Hi all,
I'm having problems changing the code on my veritas 8 alarm system.
At the moment the code is 4321 but when i put this code in, the unset light flashes and then i press prog and instead of getting all the lights coming on it just gives out a dull single tone, so i cant get into prog mode to be able to change the code.
Also the code 1234 also makes the unset light flash but it wont let me enable the alarm or do anything else with this code????
Please help??!!!