Check your connections. Each MIDI component has two to three separate ports: a "MIDI In" port and a "MIDI Out" port, and sometimes a "MIDI Thru" port. In order to transfer notes from an instrument to a computer, you must ensure that your cable connects to the "MIDI In" port on the converter cable, and the "MIDI Out" port on the instrument. If transferring from a computer to a MIDI instrument, use the "MIDI Out" port on the converter cable and the "MIDI In" port on the instrument. Connect the USB connector on the converter cable to any available USB slot on your computer.
Move your USB connector to a different USB port, especially if using a controller keyboard that requires a USB power source. A weak-powered USB slot may be unable to carry the power of your instrument, so try switching to a different port to resolve the problem. Try using a USB port directly on your computer (indicated by the three-pointed trident symbol above each port), and not an external USB hub.
Check your software settings. In order to transfer MIDI via USB, you must use a program that reads MIDI notation, such as Pro-Tools, GarageBand, Audition, Logic, Reason, Cakewalk or Cubase. Open your "Options" or "Preferences" panel, usually found on your menu bar, select your "MIDI" preferences if indicated by a separate heading, and select your USB converter cable or interface from the list of "Input" sources (if transferring notation from the instrument to the computer) or "Output" sources (if transferring notation from the computer to the instrument).
Plug in a different MIDI cable. Like all audio and video cables, a MIDI cable can short out and lose its ability to transfer data. If none of the above methods resolve your problem, try replacing your MIDI cable with a new one, available at music and electronics supply stores.