Why did the control wait for a value even after I enter the value?

I wrote a program as follows:

#include <stdio.h>

int main() {
  int i, choice, a, mul=1, j;

  printf("Enter a number: ");
  scanf("%d ", &i);

  printf("\nEnter corresponding number to perform given operation:");
  printf("1) Factorial of the number");
  printf("2) Check if prime or not");
  printf("3) Check whether Odd or Even");
  printf("4) Exit");
  scanf("%d ", &choice);

  switch (choice) {
  case 1:
    for( a=i; a>0; a--) {
      mul*=a;
    }
    printf("\n%d! = %d", i, mul);
    break;
  case 2:
    for(a=1; a<=((i+1)/2); a++) { 
      if((i%a)==0) {
        j++;
      }
    }
    if(j==1) {
      printf("%d is a prime number",i);
    } else {
      printf("%d is NOT a prime number",i);
    }
    break;    
  case 3:
    if((i%2)==0) {
      printf("%d is EVEN", i);
    } else {
      printf("%d is ODD", i);
    }
    break;
  case 4:
    ;
  }
  return 0;
}

When I ran the program it ran the first few lines correctly, that is:

printf("Enter a number: ");
scanf("%d ", &i);

But then it stopped, then and there. I pressed enter a few times but it didn't seem to work. Then I just entered '1' and then it started executing the next statements. Then it asked for choice':

scanf("%d ", &choice);

I entered '1' again and then it executed that case. I don't know why the program stopped after the first scanf statement. And resumed after entering a value.

The output screen was as follows:

Enter a number: 12



1
Enter the corresponding number to perform given operation: 
1)Factorial of the number
2)Check if prime or not
3)Check whether Odd or Even
4)Exit1

12! = 479001600