The five chiefs who oversee the police departments in Andover, Haverhill, Lawrence, Methuen and North Andover pointed out their day-to-day challenges and successes in reducing crime during a Merrimack Valley Chamber of Commerce Public Safety Forum Monday morning. On hiring, Andover Police Chief Patrick Keefe said he believes younger officers reject shift work. Haverhill Police Chief Robert P. Pistone Jr., said a new work schedule is making the difference in his department. North Andover Police Chief Charles Gray said often important collaboration efforts are unseen by the public. But, unknown by many, were the Haverhill officers who came to cover the North Andover police station when North Andover officers were otherwise engaged.