Try reloading the page and restarting the player by clicking the play button.
If you are using a mobile device, try switching to a Wi-Fi connection instead of using a cellular data connection.
If that doesn’t work, then your internet connection may not be fast enough to reliably play the program on your device. Sometimes this can happen if you are using a mobile device without strong Wi-Fi connection or during times of congestion (for example, when your neighbors are heavily using their internet connections in the evening).