Wii settings includes sensor bar location (above or below TV). Make sure that is setup correctly.
Also, this is a long shot, but maybe for some reason the Wii isn't supply the correct power supply to the sensor bar (if so, replacing the sensor bar won't help). Try sensor bar options that don't require power from the Wii (either wireless sensor bars or candles). The first party wireless sensor bars work great and go for less than $10 at GameStop. If your Wii runs to multiple TVs like mine does, it's really nice to be able to move a wireless sensor bar from one TV to the other. Batteries last forever because it doesn't really use much power.
Lastly, it probably wouldn't hurt to reset the connection between the controllers and the Wii and sync them back up from scratch.