Though we are constantly improving the flexibility of the kind of content that works within 1SE, we are not yet prepared for every possibility.

For now, try dropping the problematic video into Apple's iMovie app and then re-save it to your Photos app. 

iMovie acts as a great reset button for "trouble" content.
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If you're using 1SE on an Android phone, this is the article you're looking for.

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Tags: #distorted #stretched #enlarged #cutoff #narrow #shrunken #small #dimensions #resized #wrong #skewed