The Aspect Ratio of the file is incorrect.
The aspect ratio is a measurement of the height in relation to the width.
The height and the width are measured in pixels. An HD file is 1920 pixels wide and 1080 pixels high.
For example 1920/1080 = 16/9 = 1.778
Re-export the file from your edit suite using the correct format parameters.
Our Peach Export Guides can help you ensure your settings are correct when exporting files.
The QC report will display the Aspect Ratio of the file as a decimal.
1.778 = 16x9
1.333 = 4x3
1 = 1x1