Most messages we get are about a stream that will not play, and most of those have one of three causes that you can confirm yourself in about a minute.
Run the address through the stream checker. It tells you whether the source is dead, whether it refuses cross-origin requests (which a browser will not allow us to bypass), or whether the manifest is malformed. The playback guide explains each case and what can be done about it, and the FAQ covers the shorter questions.
If the checker says the stream is fine and it still will not play here, that is a bug worth reporting, and the section below says what to send.