If your Joy-Cons won’t sync to the Switch, start by resetting them. Press and hold the SYNC button on the Joy-Con (next to the strap connector) for 5 seconds, then release. Reattach them to the console—slide firmly onto the sides until you hear a click. If wireless sync fails, try syncing via USB: connect the Joy-Con to the console using a USB-C cable, then press the SYNC button. Check the battery level—low battery can prevent syncing, so charge the Joy-Cons for 10 minutes. Update the Joy-Con firmware: go to 'System Settings' > 'Controllers and Sensors' > 'Update Controllers'. If sync issues persist, restart the Switch by holding the power button for 15 seconds. For persistent problems, the Joy-Con’s Bluetooth module may be faulty—contact Nintendo Support for repair.