The Commissioner of the Met Police says she believes a cycle of distrust between communities and the police, is the reason why witnesses are failing to come forward.
Speaking to the London Assembly, Cressida Dick admitted her officers were struggling to bring some violent offenders to justice.
The Commissioner's comments on witnesses came as police investigating the murder of a teenager in Penge urged those with information to come forward.
It's exactly a year since 17 year old Michael Jonas was stabbed to death in Betts Park.
Earlier this year charges against six people were dropped because of a lack of evidence.