s-function in external mode doesn't work

I've created a C S-function that works in normal simulation mode. The S Function calls functions from a DLL to control a C2-Tactor. When I run in external mode (for use with a QUANSER QPidE DAQ), there is no output to the C2-Tactor. Building and running the model produce no errors. The intialization/termination routines appear to work fine. Only the specific DLL functions in the mdlOutputs routine do not seem to work. Does anyone know how to fix/debug this issue?