Going into enemy turf either no gang members spawn at all or killing 10 isnt enough to trigger a war.
Got to make sure they are exactly within the zone. The map can be hard to tell sometimes. I found that eventually if you stop enough cars gang cars will start to roll through, and shooting a couple of them through the window spawned in some gang members. Hardest one was the motel from the main missions as its mostly off-road.
Enemies in waves often spawned in the other teritories. If you leave the flashing one for to long it will warn you before it fails.
Only downside to this is you'll get a lot of heat pretty quick,
Not sure if its a glitch, but as soon as you start a gang war your heat disappears anyway and you can seemingly kill gang members in front of cops for no heat, but it has to be a kill, not blowing up the car etc.