Sometimes files can fail to convert. When this happens you'll see a red exclamation mark.
There are many possible reasons for this to happen. The most common is a corrupted file or an unsupported codec.
The best thing to do is contact us and send us the log file. You can find the log file by going File > Reveal Log File.
Also include as much information about the file as you can and when the error occurs.
Sometimes we can fix these errors but in some instances the file is corrupted and just cannot be converted.