@Alchemy 

I think what you may have is the extender is named the same as your main router, and what is happening is the available wireless devices are still connecting to the router. A wireless device will switch depending on what the device itself determines, either signal strength or until the signal is gone. 

If you know you have wireless devices near the extender, but yet they still are connected to the main router, change the extended wireless network name to be different from your router so you will know which network is which.