iMovie 10.1.4 split clip function grayed out in timeline

I am unable to split some - not all - of the clips I have imported into my iMovie project.

I am using iMovie 10.1.4 on a MacBook Air.

I seem to have this problem only for certain clips in iMovie, and I don't know why, but when trying to split the clip at the playhead in my iMovie project, the "split clip" option is grayed out, whether I am right-clicking on the clip, or using the Modify pull-down menu. Keyboard shortcut also does not work. I have tried deleting the clip from the timeline and pasting it back in from the project media pane, but the same thing happens. I even re-imported a clip to see if that was the issue, but I still have the same problem.

Any help - or even just an explanation of why this is happening - would be appreciated. I've attached a screen shot to illustrate the problem.