Pop the head off the mount and make sure all the contacts are clean. Make sure the magnet is passing where it is supposed to (there is usually an arrow or line on the pick up showing where the magnet needs to pass) and get it really close (like almost touching) and see if it picks it up. Changing the battery would be the next step.

And the unit doesn't care which way the wheel travels or which fork leg it is mounted on. Every time the magnet passes it momentarily closes a contact.