The XMPxxx series media players support various levels of H264 through to level 4.
Higher levels may play in some circumstances without issue, however when combined with other factors playback will not occure.
Because of this it is highly recommended that all H264 video clips are level 4 or below.
You can check what level a clip has been encoded to by following these instructions.
1. Download the attached program (mediainfo.exe)
2. Install this application
3. Once installed right click on the file you wish to check
4. Choose in the menu MediaInfo
5. In media info, choose view in the top menu and change this to text
6. Under the video heading, check for format profile
This will either be a number or something like High@5.1
The 5.1 or other number is the level, this should be below 4.