Spectra is not computing any values for large sparse matrix?

In a C++ program I computed a large sparse matrix energy_mat which I know is symemtric.

I am trying to compute its condition number by getting the largest and smallest eigen values like this:

    Spectra::SparseGenMatProd<double> op(energy_mat);
    Spectra::GenEigsSolver<Spectra::SparseGenMatProd<double>> eigs(op, 3, 6);
    // Initialize and compute
    eigs.init();
    int nconv = eigs.compute(Spectra::SortRule::LargestMagn);
    // Retrieve results
    Eigen::VectorXcd evalues;
    if (eigs.info() == Spectra::CompInfo::Successful)
        evalues = eigs.eigenvalues();

    std::cout << "Eigenvalues found:\n"
              << evalues << std::endl;

I see:

Eigenvalues found:

I.e. the array is empty. However I know that energy mat is symmetric (I checked) and I am later using it to solve a linear system of equations, so it's numerically sound (it doesn;t have nans or infs).

Why is spectra fidning no eigen values?

Increasing the number of iterations from 6 to 40 seems to have fixed the issue.