iMovie - Can't change length of cross dissolve

Transitions attach to (overlap) the residual media in a clip. Residual media is that media that remains, but is invisible, when you trim back a clip. So, to increase the length of the cross-dissolve trim back a few frames of the clips on each side of the transition. That will leave more residual media to which the transition can attach (overlap), thus allowing you to increase the duration of the transitions. Trim back as much as you need.

If you do the above and the transition still won't lengthen, then try deleting preferences and see if that cures it. To delete preferences open iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen iMovie in your old library to get back to your projects. Now see if you can lengthen your cross-dissolve transition. Deleting preferences is a safe procedure and will not cause loss of data

If still no luck, try putting your media into a newly created project and see if the transitions work correctly there.

-- Rich

Yes, that "not enough media available" message is very confusing.

I should mention, that the residual media issue only applies to video clips. Still clips have no

residual media. Trimming a still image shortens the duration (display time) but the entire

still image remains visible. Nothing is removed. This is unlike a video clip where trimming it actually removes (makes invisible) the trimmed footage. So, when one cannot lengthen transitions between still images, one would drag out the still image to increase its duration. Whereas with a video image, one would drag toward the middle of the clip to shorten its duration but increase the residual media. Confusing, for sure.

-- Rich