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.

Patricia Schofield
May 2012