After I unpair my iPhone from the earbuds and then re-pair the iPhone with earbuds again, the phone can't discover the earbuds. Why?

After you unpair the iPhone from earbuds, remember to go to "System > Bluetooth", tap "Forget this Device" to unpair all the Bluetooth devices. . If you do not do this, the earbuds won't be discovered the next time you try to pair them with your iPhone.