9800 Alarm goes off straight away?

Hi
I am a newbie so bear with me.

I have a 9800 Alarm set at Zone A which covers all zones. So when I come back into the house, it goes off without any time warning i.e. I need to run to the alarm and key in the code to prevent it going off.

Now sometimes I get it in time but it still goes off. How do I reset this.

Is it cos this is being part set?

Any advise / help / comments welcome.

Thanks