Buffering issues are common. There are two main causes. The first cause is that the songs clients are trying to stream are ripped at a bit rate lower than the stream setting. The second cause is that the person tuning in has too slow an internet connection.
It is the upload if all those streaming cannot keep the buffer full. It is a download issue if a singular person, or only some, tuning in cannot keep the buffer full.
You can check to see if the bit rate is set to the same rate on your encoder and stream setting. To correct different bit rates, reset the encoder to the same setting as the stream setting.
You can also tune in yourself to see if you have trouble buffering. If you have buffering issues, again it is the bit rate or your internet connection. If it works well for you but not another listener, then too slow an internet connection in that listener's download is causing the tuning in problems.