Faxes (TIFF) are blank on Windows Server 2003 Fax Server when receiving from clients who fax using their PC's

I have a client who has a Windows 2003 Server that is also acting as their fax server. The server has a Brooktrout 4-port fax card installed, and is using 3 of those ports. It is configured to receive faxes on all three of those ports, through the Windows Fax Service. It is configured to put those faxes into shared folders on that server in TIFF form.

If my client has people on the outside fax to their server using regular fax machines, the faxes receive fine and the TIFF files are able to be read by my client through the shared folders.

If those outside people fax from their computer to the fax server, however, the faxes show as received but the TIFF files are blank.

If those outside people fax from their PC to regular fax machines that my client has, those regular fax machines receive the faxes fine.

If my client tries to fax from their PC to their fax server, the TIFF files are blank. If they fax from one of their fax machines to the fax server, the TIFF files are fine.

Does anyone have any idea why the TIFF files are showing up blank on the fax server when people try to fax using their PC's?

Thanks!