The "Autoconfigure" feature works by rapidly reconfiguring the port for different baud/parity/stop features and sending a two-byte "Echo" command (0x10 0x05). When the controller responds to the Echo command, RSLinx sends a longer command to confirm the BCC/CRC error checking method and determine the Device Type.
Many add-on serial ports cannot keep up with the rapid reconfiguration changes. The best USB/RS232 device I've used is the Keyspan USA-19HS; I've bought a half-dozen of them because I give them away to users who are having this sort of problem.
Remember that Autoconfigure is convenient but not required; if you have the Offline file you can enter the speed/data/parity/error checking/Device Type values directly into the driver.
To run DF1 Full Duplex protocol without handshaking, you only need Pins 2,3 and 5, with pins 2/3 swapped between the PC and the controller. The 1747-CP3 and 1756-CP3 cables from Allen-Bradley are ideal, but any straight serial cable with a null-modem adapter should work.
Another possibility is that your offline file doesn't match the port configuration that is actually running in the controller. SLC controllers can be configured to run ASCII, or DH485, or Modbus, or even to shut down the Channel 0 port.
The last possibility is, of course, that the port has been damaged. They're pretty durable, but I've seen SLC-5/0x ports get damaged by voltage spikes (non-isolated radiomodems are notorious) or just by physical abuse.