c language scanf - fflush(stdin) - doesnt work

When I use scanf more than one time the program do not wait for another input. Instead it exits

I learned that I could put a blank space before the conversion specifier in the scanf-function - yes that solved the problem and I guess that has to do with the inputstream, that is - if its a newline character in the inputstream the scanf will consume it immediately.

scanf(" %f", &value);

But if its so - why could I not use the fflush(stdin) instead? I have tried but it doesnt work.

#include <stdio.h>
int main()
{

  float value;
  char ch;

  printf("input value: ");
  scanf("%f", &value);
  fflush(stdin);
  printf("input char: ");
  scanf("%c", &ch);

  return (0);
}