Hi, Ingrid,
Assuming that you are using iMovie 10.x.x, the first thing to try is opening iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box that appears. Sometimes that cures glitches.
If that doesn't solve it, try exporting from your iPhone into the Photos app and from iMovie accessing your vids by clicking on the Photos Library item in the iMovie media screen sidebar. Alternatively, you can import the vids to a folder on your desktop with the Image Capture app on your Mac, and from there drag the vids into iMovie, or use the import button to get them in.
Should that not work, if you are using iPhone 7 or later possibly you are exporting in the new HEVC format, that would happen by default. If so, update your iMovie version to 10.1.8 if you haven't already done so. That version was designed for High Sierra and can handle HEVC. Also, you can change your iPhone export settings from the default HEVC to the standard H.264 by going on your iPhone to Settings/Photos/Transfer to Mac/Automatic.
-- Rich