First, update to High Sierra and current iMovie version 10.1.9. See if that makes any difference.
Try also exporting from your iPhone to your desktop using the Image Capture app on your Mac. See if that captures all of the vids. From your desktop you can drag them into iMovie as needed. Another way is to import into the Photo app, and see if all the vids appear. From there you can access the vids from the Photo Library icon in the iMovie media sidebar.
It is possible that your vids from iPhone X are in the new HEVC video format and iMovie is having some problem with it. If that's the case, you can go to your iPhone camera and video settings and choose to encode in "most compatible" format instead for future vids. For your present vids, on your iPhone go to Settings > Photos and in the "Transfer to Mac or PC" section, uncheck Keep Originals to ensure the media are being converted when importing to your Mac.