After mounting the strings each one of them will lose tension unequally. You should therefore continuously ensure to keep each string tuned to the correct pitch. On no account should you over-correct the string.
Please check the position of the bridge. The position of the bridge has an important impact on the sound quality of your instrument. During string mounting, the bridge can tilt towards the scroll. When you have mounted a new string it is especially important that you pay attention to this and correct the position of the bridge, if necessary. If the bridge itself has been crafted correctly, then the best results will be achieved by tilting the bridge slightly in the direction of the tail piece end.