It doesn’t matter how much media is in the timeline. What matters is how much media for each clip is not in the timeline, how much extra media you have, usually called handles. Zoom is n a bit and open the precision editor by double clicking the edit point. Here you’ll see much handles you have and you can slip the shots to lengthen the handles and lengthen the transition.
If you're dead absolutely out of media to use one trick I use is to speed ramp slower a bit of the handles. The dissolve masks the change and it doesn't get the rest of the timeline all out of whack, timing wise.
Many thanks everyone for the help. You're quite right of course - there wasnt enough of a handle. There had been before but I must have lost it during editing. Problem solved by reducing the speed of the clip down to 80% to create the handle (its just rain falling so it looks fine). Thanks again, Rich