Spent most of yesterday putting together photos/videos to create the party video

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I had the backkground music which was a spiritual song that we were practising before the party. Many guests sent me videos and photos that they had clicked at the party. I ony had to add captions to each of those photos which was really time consuming. What took my time however was a video that I had planned to add to the video I was creating and that video appeared horizontal I tried everything possible but it stayed that way Being the best video that I had received I reconciled to it and accepted the fact that whoever sees the video will have this part of the video in a horizonal mode. What could be the reason for this?