I assume that you are using current iMovie 10.1.11 and Mojave. If not, update to those. Also, are you exporting from the Photos app to your desktop? If so, how are you trying to import them into iMovie? What is the full audio and video format of the files?
In any event, iMovie fully supports M4v and Mp4/AAC. If it doesn't recognize or import them a corrupt preference may be causing that. Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your preferences. Now see if your videos are recognized and importable.
Also, the recommended way to import videos into iMovie from the Photos app is to click on the Photos item in the iMovie project sidebar, and drag the photos in. The Photos library need to be designated as the System Library in the Photos preferences.
-- Rich