Flash Drive only mounts through USB Hub
I had a similar thing with an old pen drive that was faulty. 
First thing I would do is go to the Console and check whether the OS complains when you insert the pen drive. In my case I would get a lot of I/O errors related to the USB device.
Reformatting and partitioning the thing with Disk Utility solved my problem (but I never trusted the drive again, I have to say)