If the video was recorded on a mobile device, they save all vidoes in a landscape orientation but then included a property that tells video players to rotate portrait videos by 90 degrees when they open them. Unfortunately, FlashBack Pro does not currently support this property so will not perform the necessary rotation. 

The only suggestion we could make is to re-encode your video into a true portait orientation before importing it into FlashBack Pro. You can do this using Flashback Vodeo Converter or by trying one of the guides in this article - https://www.howtogeek.com/howto/14751/rotate-a-video-90-degrees-with-vlc-or-windows-live-movie-maker/