The Polar manual should specify a max distance from the computer to the sensor. Is your rear wheel sensor within that distance? If it is the battery in the sensor may be bad. It's normal--at least it was when I used Polar stuff--to mount the wheel sensor on the fork. As for the cadence sensor, the max distance requirement as well as the battery statement both apply as well. You have the option of mounting the cadence sensor on the down tube. You should also check to be sure that the battery in the computer is full and that both sensors and their respective magnets are well aligned.