Some things to try:-
This sometimes happen when you try to connect more than one drive using the same usb port? Have you tried connecting a drive that does work, leaving it in that port and then putting the flash drive that is not being recognised in another usb port? It should then be allocated a drive letter.
When you insert your flash drive and go to disk management can you recognise that drive by the size shown even though it has not been allocated a letter? If so are you able to right-click on the drive description in the lower right-hand pane and select change drive letter?
Insert your flash drive. In Device Manager can you click on "show hidden devices" by clicking on "view", delete any usb devices that you know are not connected and sometimes your device will show. If it doesn't remove it and re-insert it.