If it is a wireless computer, I have three suggestions:
1) Check to be sure that you told the computer which sensor to read. I am not sure if all of the wireless computers require this, but my Cat Eye required you to hold the computer next to the sensor and push a button on the back of the sensor to tell it which sensor to read from.
2) Check the battery in the sensor to make sure it is not dead.
3) Check to make sure that the path between the sensor and the computer is reasonably clear. If you have gear in between the two, sometimes it will block the signal.
If it isn't wireless, try these ideas:
1) Check the battery in the sensor if it has one that is separate from the computer.
2) Check the entire length of the cable to see if any section of it feels kinked or unusually limp like the wire inside has been broken.
3) Check to make sure that the sensor is close enough to the magnet...i know you said it is at the spec, but try moving it closer...maybe the magnet is just weak.
4) Make sure the flat face of the magnet is facing the sensor...I know it probably is, but I had to say it just in case.
5) Make sure that the computer isn't on "avg speed" setting, which would show zero for a while when the wheel first started turning.
Other than that, I would say take it back to the shop and ask them what is wrong or get a new one to see if it was just a defective model.
