how to re-import clips that were imported incompletely (clips are green)

I have imported about 50 clips. After a while I saw that the first 20 or so had been succesfully imported and I started to move them to the timeline. However I did not notice that the import of the rest of the clips was still busy. As a result the import of the rest of the clips failed, the clips are green. When I play them only the audio works.

I tried several ways to re-import them, first I delete them from the library and then for instance I start a new project, new event, omp[prty again etc. But nothing works : the clips remain green. For some reason iMovie cannot import them again. Who has a solution ?