Transitions hook on to the invisible residual media that remains when you trim back a video (not still) clip. You may not have sufficient residual media to support a transition longer than 1 second. Trying trimming back a little bit the video clips on each side of the transition and see if that solves it. If the transition is between photos, as opposed to video clips, the photos need to be of sufficient duration to support the transition. If that's your situation, try increasing the duration of the photos on each side of the transition.
You also can set the default duration of transitions that appear in the project by clicking on iMovie/Preferences and making the adjustment in the box that appears. The adjusted setting will apply to future created projects.
-- Rich