In Expression and Obsession:
Remember that the B dimmers are actually DMX512 addresses 257-512. For this reason the console should be set up to have enough dimmers to cover this range (Number of physical dimmers + 256). It is usually easiest to just set the number of dimmers to 512.
On both the Expression 2/3-Line and Obsession I/II consoles, this is done via Setup -> System Settings -> Number of Dimmers.
Note: that if there are fewer than 256 dimmers when you enable a port for Dimmer Doubling, no error will be displayed.
In Eos Family:
An Eos family console defaults to having a Dimmer Double Offset of 20,000. This is due to the fact that it is much more common to use entire universes of data in this line of console than it was in the Expression family. Verify the B side of your dimmer is patched and that should correspond to the appropriate address in the 20,000 range.