C++ rand() not generating random numbers [duplicate]

I need to randomly choose one of two options several times in a row. I am using std::rand() function for this, here is the code:

string obfuscate(string str)
{
    int i;
    int brackets = 0;
    unsigned int coinflip;
    srand(time(NULL));
    for (i = 0; i < str.length(); i++)
    {
        if (str[i] == '{')
            brackets++;
        else if (str[i] == '}')
            brackets--;
        if ((str[i] == ';' || str[i] == '{' || str[i] == '}') && brackets > 0)
        {
            coinflip = std::rand();
            cout << coinflip << endl;
            if (coinflip % 2 == 0)
            {
                string substr = str_gen(brackets);
                str.insert(i + 1, substr);
                i += substr.size();
            }
        }
    }
    return str;
}

The result I get seems very weird. I print every rand() output for debug purposes. Here is what I get However when I debug the program step-by-step, the generated numbers are pretty random. I also tried this cycle in the same project:

int i;
    srand(time(NULL));
    for (i = 0; i < 20; i++)
    {
        unsigned int coinflip = rand();
        cout << coinflip << endl;
    }

The output is pretty decent too. So, my question is, what am I doing wrong in my main cycle, that the numbers are not generated randomly?

Note: I am NOT USING srand() inside a loop, that's not the case here.