usb device feature isn't working in case of external power supplied into the board
VBUS shouldn't be "float". If powered from USB cable, the VBUS should go to pin P1[30], configured as VBUS, with proper level translation.

If on internal power, this pin still should still be related to USB VBUS on the cable. The VBUS wire is an important side-band signal in USB protocol, it should be logical HIGH for the USB device to start operating. 

You need to check your "own designed board" against manufacturer's demo platform.