Why isn't the external sounder going off when the part set alarm goes off?

I have installed a veritas r8 alarm in a clients house with a PIR in the garage and 3 PIR's in the house. Everything is working fine on full set but when the alarm is part set for the garage PIR the external sounder (bell box) doesn't sound when triggered. How can I get it to work? The alarm in the main panel sounds and a sound bomb I have installed in the garage works just not the main bell box. It's only not working on part set. Please help