The channel really doesn't matter - As long as the SSID and security settings are the same, devices should recognize it as a valid access point to connect to and connect to it if its signal is stronger. In fact changing the channel to match the main AP will cause interference and slow things down.
If devices aren't roaming, it's probably due to the device configuration. Some wireless adapters have a "roaming sensitivity" setting that lets you configure how "eagerly" the adapter will switch to a stronger access point.
If your adapter has such a setting, in Windows you can probably find it in
Network and Sharing Center->Change adapter settings->Right click on wireless adapter->Properties->Configure->Advanced tab
Look for a property similar to "Roaming sensitivity".
If you can't find one, try installing the manufacturer's driver (instead of using the default Windows driver). If it still doesn't appear, it's probably best to just let it be - it probably means that the difference in signal strength between the main AP and the extender isn't big enough for computers to switch to the extender.
Just curious though - what software are you using to check which access point your computer is connecting to?