Unless your pond builder was careful to put a layer of good topsoil over the top of all the soil they used in construction, it is likely subsoil with low "nutritional value" for plant growth. If you can get topsoil spread over the area now, all the better. An aggressive program of fertilizing may help otherwise. To get new grass to grow, it needs regular moisture. Not just when a big rain comes and washes seeds away. Have you put some barrier over the seeds when sown? Straw or some covering will hold in moisture and help hold things in place.
