Connect via ssh, run the following command in the shell and check what happen if you replug USB:
sudo dmesg -wH
You will see errors, something similar to enderbender or nothing.
Nothing or errors means that you have a hardware issue or something is incompatible. The pi’s not allways like USB devices^^ You can try another cable or a shorter cable 
The enderbender message means usb device has been detected
Good luck!
PS: If you see enderbender like messages you can also reach your serial device via: /dev/ttyUSB0 (or something similar, see your dmesg message). Sadly less stable if the connection is lost and replugged because it can be renamend to ttyUSB1 then. But i think it fine and will work forever