ADT - Alarm not functioning

My alarm was installed by a ADT approved installer. Our request was simple - Make it pet friendly. The alarm is constantly going off, moments after we left the house. Certain sensors doesn't work properly and someone actually walked up to my front door without any alarm response. When I phoned ADT to complaint about the supplier, they passed the bucket. I must add that ADT\s response to any alarm is very good. They phone immediately and the armed response is there very quickly. I just need someone to rectify my system and give good advise regarding extension of the system."