Look at a wiring diagram for a null-modem connector. The TX and RX lines are not looped back. They're swapped. Lines that are looped back and shorted include CTS and RTS. If the empeg connector isn't wired right (which, again, I seem to recall, but can't find documentation for at the moment), it might be looping back ground to CTS, which wouldn't raise it, and the OS would refuse to send data out the TX line, possibly depending on the options with which the port was opened.

Honestly, I hadn't done a lot of research into this to see exactly what happens in this configuration. I didn't think it was a difficult concept, and just required someone to chime in with better search-fu than mine at the moment to confirm that my memory of the incorrect wiring was correct and to see if it had any effect on the RTS or CTS (or any other signals, like DSR and DTR, possibly) lines.