I had similar issues with my garmin cadence meter. Due to the odd shaping of the chain stays, I had issues getting both my cadence and speed meter to read at the same time (while trying to leave as much space from the cadence meter to the spokes)

In the end, i read the fix-it with using rare earth magnets and put two of the magnets on crank (in the pedal shaft) and superglued two onto original magnet and put them on the outer spoke of the rim.

Problem solved for me.