Good idea, will do this now.

Three years ago I witnessed an aggravated racially motivated assault on a passenger at London Bridge (between well dressed middle class commuters during the morning rush hour)which led to the victim being covered in blood. I phoned the police to give evidence.
No one returned my calls. Eventually after hassling them someone came to take a statement three months later. I saw the offender daily on the train and advised the police. It took a further 6 months for the police and a lot of persuasion from me to 'stake out' (exciting stuff) FH station and to approach the assailant. They said they did not have the personnel available to do this nor to take the statement. They were arrested but it took a further 7 months to get to court, which I attended to give evidence. A suspended sentence was given out to the offender.

My point is that it may be that there are people out there with evidence and information but there is no one there to take it. It can be as basic as that. It was hard work at the time and not everyone would be bothered. Lets hope that things have improved.
I see no reason why these gang members cannot be identified other than lack of exposure.