Here are some steps to troubleshoot playback issues:
1.) Verify that the source stream can be played well. Try to do playback on well-known players, e.g. JWPlayer for RTMP, VLC for RTSP.
2.) If the source stream works fine, try to feed it to EMS and see the logs. I suggest running EMS on console mode so you can see the logs real-time. Most common playback issues deals with codec incompatibility, or connectivity issues. The logs will indicate these problems.