Can't reverse clip in iMovie

Am using iMovie 10.0.9 and the function of reverse a clip is no longer active nor is slow/speed function. The movie clips are from iPhone 4s + 6 and some from Sony Handycam AVCHD.

I had a clip which I could make reverse but then I deleted it from the Timeline and since then the whole function doesn't work.

I have tried:

• to convert/export the clips to .mov .mp4 .m4v but nothing helps
• to delete iMovie (using AppCleaner) and reinstall it
• I have made sure that the clip is on the timeline

What can I try out? 😢

I have absolutely no idea what is going on other than it might be some silly little thing I forgot to do....