Are you unable to adjust at all, or simply cannot make than *longer*?
The red edit points indicate that there is no extra footage in the clip, so you can't extend the transition. This is normal. You need "handles" - extra footage at the end or start of a clip to make a transition.
One way to easily see the available handles is to open the "precision editor". Double-click the edit point to activate it. If you don't see extra footage after the edit point for the first clip, or before the edit point for the second clip, you can't extend the transition.