Not sure what is going wrong in CallTwainProc
But at the end of your SetBrightness function you are not freeing memory on success.
if(CallTwainProc(&m_AppId,&m_Source,DG_CONTROL,DAT_CAPABILITY,MSG_SET,(TW_MEMREF)&twCapability))
return TRUE;
GlobalFree(twCapability.hContainer);
return FALSE;
} Jim Watters