The LAPD has been trying to settle things down, but flooding the area with officers hasn’t worked. Neither has relying on veteran gang members to manage rivalries. Or pleading with residents to report what they know or see.Until recently, those tactics helped drive crime down to a 20-year low. But gangbangers have become bolder, and the violence is less predictable. Residents fear gangs more than they trust the police.The surge in shootings over the last few months reveals how little we know about what drives crime — and suggests the limits of our holy grail, community policing.Half of the city’s August homicides occurred in South Los Angeles, and 70% of those are believed to be gang-related. For many residents, that feels like a return to the bad old days.