C++ :: Randomly Generating Probabilities - Function Stuck

I've written some code to randomly generate probabilities:

int Queue::car_per_hr() {
srand( time( NULL ));
int prob = rand() % + 101; // Probability in percentage from 0 to 100
int cars; //no of cars per hr

[Code] ....

So it checks through the range of possibilities: 0-20%, 21-40% etc. I then have this to implement it:

#include <iostream>
#include <ctime>
#include "queue.h"
using namespace std;
int main(){
Queue car_wash;
int NO_HOURS;

[Code] .....

For some reason though, everytime I call the car_per_hr() function, the probability just stays the same. It's output is always locked to the same thing it had to begin with?