Issues with FCP X Transitions - The Chevron transition doesn't work

I'm using FCP X version 10.4.8 and having an issue with the Chevron transition. I added this transition to my project at an early stage and it plays fine. For some reason I am now unable to add an additional Chevron transition anywhere else in the project. I can see the transition occur if I slowly scrub through that point on the timeline, however, it doesn't playback in real time. The timeline video plays a if there is a simple edit between two clips. All of my other transitions play fine and new ones can be added to the timeline.

I deleted my preferences and relaunched FCP X. The problem persists.

I started a new library, created a short timeline and added three clips. I dropped the Chevron in between the clips and the problem is still there.

I'm editing off a 3 TB external media drive and still have 663 TB free.

Are there technical issues with this version of FCP X or a problem with the mac computer I'm using?

What could be causing this problem with Chevron and how can I fix it?

"I'm editing off a 3 TB external media drive and still have 663 TB free."
Do you mean 663GB or 663MB?

What is the duration of your transition?
What format is the video you are editing?

Rather than preferences, which have nothing to do with this issue, delete all the render files for that Library, relaunch, see what happens.

Hi FCPX.guru,

Thanks for responding.

Good news! After reading your post I tried a few simple render tests in 1. a brand new library timeline and 2. the timeline of my current project. After rendering the chevrons played fine. I find this unusual because most of my transitions play without rendering.

To answer your questions...

What is the duration of your transition? One second.

What format is the video you are editing? I've tried a few. The clips with the transition that works are AVCHD / 4K. I did try a test with some standard HD footage.

I greatly appreciate your response.