First start by clearing your cache and restarting your browser.
Read over our upload guidelines to make sure your file is up to our specifications.
If everything looks good, we would suggest cancelling the master, deleting it, and re-uploading the original file completely. Sometimes an upload can be corrupt.
Lastly, check the settings in your library and ensure you have Mastering Preferences turned off; Mastering Preferences lets you set your preferred settings and bypass the preview entirely!