When installing a wireless computer the first thing that I do is to install the wheel magnet and sensor. Locate them as high on the fork as you can. Then I hold the head unit close to the sensor and spin the front wheel to be sure that I get an MPH reading. Once I'm sure the computer is working I spin the wheel again and hold the head unit various places on the handlebar and stem, making sure that it works before mounting the head unit on the handlebar.

Wireless computers have a limited range and they also seem to be somewhat line-of-sight. Sometimes if the fork blade or something gets in the way they aren't able to pick up the signal.