I have put three folders on the USB stick named after three themes. Christmas, Wedding and Birthday. I have put 20 WAV files into each folder. I press BGD and Song and I can see the 3 folders. Only 6 of the 20 WAVs can be seen in the display. Selection one and pressing BGD again plays it perfectly and then stops. I know how to set up random play order but I can only get one WAV to play. The manual does not help me. What do I need to do please?