It could be a problem with the sensor bar. Because you have to point the Wii remote towards the sensor bar, and if its up high you could be pointing it to low and if its down low you could be pointing it to high. Hope this helps.
I am having the same issue although i know my sensor bar is plugged in!
What should i do?
While your Wii console is on, try looking at the sensor bar with a digital or mobile phone camera and if the sensor bar is functioning properly, you should see a set of lights on either end.:)
If no lights, then you likely need to purchase another sensor bar, sorry.
But if there is lights, then it might be another problem either with your Wii console and/or the Wii remotes, which further troubleshooting to figure out might be needed.