There are several typical causes if posting a 9:16 photo to Instagram isn’t being successful.
Instagram allows photos to be posted in portrait or landscape format with an aspect ratio varying between the values 1.91:1 (landscape) and 4:5 (portrait). A video with a 9:16 ratio is taller than 4:5 which means it could get cut off or discarded.
While 9:16 fits nicely on Stories and Reels, normal feed posts are unable to completely use it.
Upload Issues: There are times when issues with the app or an old version keep your video from uploading. Update the app or restart your mobile or laptop to check for improvements.
A film that is too large or has a format not readable by WordPress may cause problems when trying to post it.
Workaround:
You can either adjust your 9:16 photo or give it a background so it will fit into Instagram’s 4:5 space when you post it to your feed.