How can I fix my Optima XL6 Alarm?

I have recently moved into a bungalow which has an Optima XL6 alarm installed, and am experiencing a problem as follows:
When setting the alarm and exiiting the front door the alarm correctly beeps for approximately 1 minute, when re-entering the front door the alarm beeps continuously and the alarm bell does not work, giving access to the two bedrooms. Shock sensors on bedrooms/bathroom windows work OK. PIR sensor in the lounge works OK. My query is, why does the alarm bell not work on entering through the front door.