|Sometimes a USB port is used for a specific device like USB internet modem whereby that port is "reserved" only for that device and no other usb device will work in it.|
Sometimes a user may have used a usb port with a high powered device and has caused an "overload" and has either temporarily disabled that port (restart usually fixes the problem) or has shorted the port and is no longer be able to be used.
Sometimes a usb port is damaged by a user trying to force in the device upside down and the connectors are damaged making the port unusable.
If there are other usb ports in use, there may not be enough allocation power available, try removing other devices. All the USB ports are allocated a maximum amount of power so if there are too many devices attached, a new device may not be recognized.
Also if there is an option in the bios, change it to enable usb 1.1/2.0 /3.0 however, if you are sure that you are not going to use version 1.1 usb then don't select that option although, if you don't then some ports may not be available for use as they are usb 1.1 specific.