Trouble understanding some unexpected behaviour in code with OpenMP

Question regarding OpenMP parallelization. I have included a stripped down version of my function below. The problem is that, the contents of the for loop are not getting evaluated for all values of uiIndex, although not always.

1. I use the buffer vec_succ_status to check if all values of uiIndex are getting evaluated. It turns out that it is not.
2. My code does not crash, it just exits from the function compute_Lagr_shortest_paths_from_source, without encountering any of the exit(-1) statements in the function definition below.
3. I am using g++ 7.4.0 version on Ubunutu 14, and every time it has failed, there is exactly one value of uiIndex that was skipped. There is no consistency to the uiIndex for which the function fails to evaluate.
4. In my main function, I included the line omp_set_num_threads(4). Apart from that, I did not set any other settings (such as scheduler type) for OpenMP.

If someone can point me to the problem I might be facing, that would be great. Thanks.

bool compute_Lagr_shortest_paths_from_source(std::vector<Robot_CF_Graph>& vec_cf_graphs, const std::vector<std::vector<size_t>>& vec_robot_groups)
{
    size_t uiIndex;
    std::vector<bool> vec_succ_status(vec_robot_groups.size(), false);

    #pragma omp parallel for default(shared) private(uiIndex)
    for(uiIndex = 0; uiIndex < vec_robot_groups.size(); uiIndex++)
    {
        vec_succ_status[uiIndex] = false;
        const auto& vec_group = vec_robot_groups[uiIndex];

        if(1 == vec_group.size())
        {
        size_t uiRobot = vec_group[0];
        vec_cf_graphs[uiRobot].compute_shortest_path("ABC"); 
        vec_succ_status[uiIndex] = true;  
        }
        else          
        {            
        std::cout<< "Tag: Code should not have entered this block"<<endl;
        exit(-1);                       
        }

        if(false == vec_succ_status[uiIndex])
        {
        std::cout<< "It is not possible for this to happen \n";
        exit(-1);
        }
    }   

    return true;     
}