Verify that the TFTP server is open on your PC. Also, make sure the file is in the root directory (from the TFTP application software menu bar, choose View > Options).
Try to eliminate extraneous networking complexities between the switch and the TFTP server. Try moving the TFTP server or setting up a new server on a network segment topologically closer to the switch, or on the same LAN segment as the switch.
Finally, if these actions fail to resolve the problem, try using different TFTP server software.