The higher you move on the fork, the more clearance you must have as the spoke moves towards the rim.

OTOH, if you have a mounting issue, try to put the magnet on the opposite side spoke and see if the sensor picks it up. I had to do that on a friend's bike the other day (not much room to move the sensor due to it being a dual cadence/speed sensor) and being mounted on the LR chainstay.