how can i fix my accenta g4 alarm?

i have a pet friendly accenta pro alarm kit but have a problem with it. When the alarm is set and you open a door contact or trigger a sensor, the external siren only sounds for about 5 seconds then turns itself off leaving just the internal siren going which is not very loud, has anyone got any ideas what's wrong with it? the default setting is set for it to stay on for 15 minutes which is what i want.