Lengthening the clip in iMovie

What steps are you doing to change the length (duration; playing time) of the clip? Are you using the Clip Information button and duration box to do it? Are you changing the duration of a still image or video clip? What happens when you drag the edge of the clip to change its length?

The duration of video clips cannot be increased beyond the duration created when it was recorded, unless you use the speed adjustment feature to slow it down (thus increasing its playing time).

In any event, try deleting preferences and see if that cures the issue. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

-- Rich

You should be able to change duration of a clip by dragging the edge of the clip, not just from its corner. If it only drags at the top corner, to increase the length of a video clip, then possibly you have the Show Speed Editor handle displayed, that appears in the top right hand corner when one Control-clicks on a video clip (not still image) and selects Show Speed Editor from the drop down menu. Then, when you drag the handle to the right the clip will elongate beyond its original duration to slow down its speed (make it slow-mo). Sliding the speed handle to the left will shorten the clip duration and increase its speed.

-- Rich