some bikes do not have enough metal to trip the magnetic sensor
Correct. The sensor at the lights is just a large metal detector. Its sensitivity can be adjusted by the DOT, so if there's one in your neighborhood that always fails to notice you, try writing a polite letter to DOT.
Rather than using a green light trigger, which I believe is just a magnet, if you are stuck at a light that won't see you, try a couple of things.
1. Make sure you're sitting right on top of the wire, preferabley at a corner or intersection.
2. Thump your centerstand against the ground. You're moving metal right up against the sensor's wire, which may trigger it.
3. Shut off and restart your bike. Your starter throws off a very strong electromangetic field when it's in use. This field will trigger the sensor.
I almost always use technique 1, occasionally try technique 2, and have had great success with technique 3.
There's also technique 4. If the light's not triggering for you, it can be considered to be malfunctioning, at which point you legally treat it as a four way stop and go when you can safely do so.
Fred