Not sure how yours comes apart. Mine is just clamped onto the spoke. I don't suppose it has to be that magnet, though, if you have another one handy.
The problem almost certainly is that it was installed just barely close enough to work, and/or it somehow moved slightly - just enough to no longer reach. Your spokes get closer to the fork as they go toward the hub. To get the magnet closer, you will have to move it toward the hub end of the spoke. Depending on how far you move the magnet, and where it us placed currently, you may also have to move the sensor so that they are aligned.
It's all quite easily done, but you may need another zip tie for the sensor, depending on how yours is mounted.
BTW, I just looked at mine. My sensor (Sigma computer) is mounted with an o ring, so it would be easy to move. The magnet has a metal outer piece that slides on once you get it in place. To remove, you would just slide the metal piece up or down.
Hope this helps.