Why my program print the same value?

#include <bits/stdc++.h>
using namespace std;
#define CROSSOVER_RATE            0.3
#define MUTATION_RATE             0.01
#define POP_SIZE                  50          
#define CHROMO_LENGTH             150
#define GENE_LENGTH               30

int random_num(int start, int end)
{
    int range = (end-start)+1;
    int random_int = start+(rand()%range);
    return random_int;
}

string  GetRandomBits(int length)
{  srand(time(NULL));
    string bits;
        for (int i=0; i<length; i++)
    {
        if (random_num(0,1) == 1)
 
            bits += "1";
 
        else
 
            bits += "0";
    }
 
    return bits;
}

int BinToDec(string bits)
{    int val = 0;
    int p=1;
    for (int i = bits.length(); i > 0; i--)
    { if (bits.at(i-1) == '1')
        {val += p;}
        p *= 2;
    }
    return val;
}

double nr_interval(int valoare, double a, double b)
{
    double result;
    result=0.0;
    result=a+valoare*((b-a)/(pow(2,30)-1));
    return result;
}

double De_Jong(string bits){
    double x=0.0;
     for(size_t i=0; i<sizeof(bits); i=i+GENE_LENGTH)
     {  x = x + nr_interval(BinToDec(bits.substr(0,i+GENE_LENGTH)),-5.12,5.12)* nr_interval(BinToDec(bits.substr(0,i+GENE_LENGTH)),-5.12,5.12);
         
     }
 return x;   
}

int main(){
     
    for(int x=1; x<=POP_SIZE;x++)
     {  cout<<De_Jong(GetRandomBits(CHROMO_LENGTH));
        cout<<endl;
     }
    //valoare=BinToDec(GetRandomBits(l));
   //cout<<valoare;
    
    return 0;
}