Retro grouch is right. Most computers send out a narrow cone shaped beam vs. transmitting 360°. This serves two purposes, first it lower the power requirements extending battery life. Second it reduces cross talk or interference when two owners of the same computer ride together.

There are a few steps to getting a good connection.

1- try for line of sight, yes it's radio and not light, but metal obstacles interfere. If you have canti's and a short stem, mounting the sensor lower may help.
2- note the correct orientation for the sender, some only transmit in one direction, so there's a top and bottom, and you might have to switch sides on the fork.
3- know how the reed switch works. Some are single ended so work best with the magnet passing low on the sensor. Others like SigmaSport are teeter/totters and work with the magnet passing above or below, but not at the center.
4- make sure magnet distance is short, about 1/8" or less.

Lastly when setting up and testing, be aware that some sensors and computers have a sleep mode to save batteries. For example a SigmaSport computer won't begin receiving until a bump or vibration wakes it up and turns on the radio. This used to drive machanics crazy when they couldn't get them to work on a repair stand. If your's won't receive, bounce the front wheel, or in a stand give it a good shot.