If you look at the block diagram, the only controls between the send bus and the eff-send outputs are the send-masters, so if these are turned up full and you can hear a signal when monitoring the send bus, it must be present on the eff-send outputs.
How are you determining that you are not getting a feed to the external unit?
Try plugging something else such as your powered speakers into the eff-send outputs.
If there's no sound, something odd is happening inside the DP machine, in which case have you done the usual fault-finding steps such as checking the firmware is up-to-date and/or doing a factory initialise.
If there is a sound, you know the DP machine is sending ok and the problem is with the external unit or its connections, in which case check what Mark and FP said. You just have to be methodical and trace the signal through step by step.