Here are a few things to check:

Not all photogates support Auto ID.
– Older Vernier photogates (shipped before mid-June 2003) were made before the Auto ID feature was implemented. Also, photogates made by other companies are unlikely to Auto ID in Vernier software. We have pictures and adapters for many photogates in our online adapter wizard at: http://www.vernier.com/adapters/

Check the software.
– Logger Pro allows you to set up older (non Auto-ID) photogates.
– If you are using Graphical Analysis 4 software, it is not possible to configure sensors that do not Auto-ID.

Check the cable.
– The cable should have an RJ-11 (phone jack) style connecter going into the gate head and a white flat BTD connector going into the DIG/SONIC port on the interface. Cables that have a white flat BTD connector on both ends work only with Motion Detectors. The correct cable is BT Photogate Cable (PG-BTD).
– Make sure your cable has six conductors instead of four. The cable that comes with the Vernier Photogate has a six-conductor clear plastic telephone-style connector at the photogate end, often referred to as 6P6C, for “6 position, 6 conductor.” The cable looks much like the cable that came with pre-2003 photogates, but the old cables had a four-conductor telephone connector.
– A four-conductor cable will work with the Vernier Photogate in Logger Pro, but the auto-ID function won’t work because the necessary connection is not made.