Discrete S-Function doesn't update inputs

Hello,

I have written a C MEX-S-Function for my model. I set the sample time as follows:

static void mdlInitializeSampleTimes(SimStruct *S)
{
    ssSetSampleTime(S, 0, 0.001);
    ssSetOffsetTime(S, 0, 0.0);
}

The input of the model is a continuous signal from a physical model. As I specified the sampling time I expect the S-Function to sample the signal accordingly but it doesn't. The S-Function doesn't recalculate. If I set a unit delay in front of the input, the S-Function works as expected but I falsly introduce a delay. How can I solve this issue?