"Clips do not have enough media for the transition" ???

To extend the transition try using the precision editor instead of trimming. Right click on the transition and choose the precision editor.

I have had this issue too while learning iMovie so I added a Title page, with Title slots blanked out, after the transition. This has worked for me especially with a Fade to Black transition and then the blanked Title page...it gives the same appearance of a longer transition when the Transition duration cannot be extended. The duration of the Title page can be adjusted usually normal clipping techniques.

trimming back clips doesn't work for me either. and as you mentioned both adjacent clips have plenty of media (both 60 seconds long).

TRIMMING BACK FEW FRAMES is totally irrelevant, I deleted all the video clips and all the images except one and still cannot extend my 'transition' from 1 sec to 4 sec. It is really annoying to the point I am really considering throwing away the iMAC technology, which I have been using since 2009.

Trim back a few frames on the video clip. That will create some unused video for the transition to overlap onto.

When you trim back a video clip the part you trimmed away still remains in the project as unused video. It's not an issue with still clips.

It's interesting that iMovie shortens clips automatically when you add a default duration transition but if you try to extend the transition afterwards you have have first to do the extra trimming yourself. I have never understood why this behaviour was chosen.

With version 10.1.2 you now get a warning that the clips have to be shortened but in my experience it doesn't actually work and you still have to do the extra trimming yourself.

Geoff.