When you connect the 3012 board, if you turn off and on the external VBUS supply, is it able to detect the board?
Have a look at this datasheet:
Go to 'Errata' on page 47:
2. USB enumeration failure in USB boot mode when FX3 is self-powered.
■ Problem Definition FX3 device may not enumerate in USB boot mode when it is self-powered. The bootloader is designed for bus power mode. It does not make use of the VBUS pin on the USB connector to detect the USB connection and expect that USB bus is connected to host if it is powered. If FX3 is not already connected to the USB host when it is powered, then it enters into low-power mode and does not wake up when connected to USB host.