final cut pro
When you add a transition between clips, it still needs to be "rendered," or processed by the computer to permanently add it to your sequence of clips.
Sometimes when you add a transition, you'll see a green line above the transition, which means you can view the transition, but it still needs to be rendered.
Other times when you add a transition, you'll see a red line above the transition, which means you can't view the transition until it is rendered.
To render a transition, in the Final Cut Pro menu at the top click on:
(which will render all the transitions you've added to your sequence)
Final Cut then will process your transition and permanently apply it to your sequence. The time it takes to render depends on how sophisticated your transition is. More advanced transitions could take up to a minute to render.
When the rendering is completed, the red or green line should be gone from above your transition.