Stop with your front wheel directly on top of the upper right or upper left corner of the rectangle. If your back wheel is also on top of the rectabgle, that will help as these are inductive loops that are "de-tuned" by the presence of metal, so the more metal the better.
A rare earth magnet is not going to do anything unless it's a honking big magnet.
The sensitivity of the loop can be adjusted, and some may not be adjusted sensitive enough to detect a bicycle. In that case, try calling the road department in the jurisdiction where the light is located and ask them to readjust the light. I've watched the local department adjust the light sensitivity by using a bicycle wheel!
The loops around here have a small X painted on them where you are suppoed to place your front wheel which makes it easy to trigger them.