Reverse clip not woking, greyed out. Why?

Working on FCPX. Editing a project. On the timeline (filled with clips), I select the clip I want to reverse, and click the Retime pop-up menu below the viewer. All options on the pop-up menu are available, EXCEPT Reverse Clip, that is greyed out (unavailable). I try selecting other clips randomly, and the same happens. I cannot reverse any clip.

I tried opening other projects I already edited, and on those, the Reverse Clip works fine.

The project I am editing right now is a duplicate of those other projects where the Reverse Clip works (I prefer doing duplicates just to make sure all settings are the same). The footage in question was produced by the same camera (iPhone) and same settings used on the footage edited on other projects.

I tried importing that clip to a different (older) project, and the Reverse Clip feature was available (so the problem is not with the clip). I reversed the clip as I wanted, then I copied/pasted it to the project I'm working now. The clip was copied reversed... and now, the Reverse Clip command on the menu is available ONLY for that clip. The other remaining clips on the timeline still have the same situation: the Reverse Clip command is greyed out, thus, not available.

I have several projects edited, and never faced this problem before.

Any clues?