Transitions in iMovie

I wasn't able to replicate your situation in my older iterartion of iMovie 10. I placed a series of photos in the timeline with durations varying from 1.0 second to 4.0 seconds. Then I inserted a 2.0 second wipe left transition between all photos. The transitions played as they should.

Sometimes when iMovie acts in an unexpected manner it is because of a preference issue. Try deleting (resetting) the iMovie preferences by opening 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 library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

If deleting preferences does not cure it, try creating a new project and see if the transitions work properly there.

-- Rich

At the places where the problem is happening, are the clips photos or are they videos?

You might try redownloading iMovie. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications

folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. ( You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.

-- Rich