A recent commenter on a post said they couldn't get the loop detectors at traffic lights to work for them. There is some confusion over how these work. The loops detect metal, not weight. If you can see pavement cuts forming a box, or two parallel lines, you should position your bike over the righthand pavement cut. This is where it's most sensitive. If you see three parallel pavement cuts, like on Clinch at Henley, you should position your bike over the center cut.
If you try this and still can't get it to change (give it a couple of minutes--it's not instantaneous), you can let me know which intersection (email@example.com) and I will get the City (or County, but I don't know that the County has many intersections with loop detectors) to look at it and adjust the sensitivity.