transition duration problem

I'm super new to video and FCP. I've been working with FCP for a month and everything has been going well, until today. I'm trying to add a normal transition between clips and the duration I'm getting is for only .04 seconds. I found where to set the default duration in preferences (I put it at 2 seconds) but the transitions are still only .04 seconds. Not only that but I can't change the time of the duration like I used to be able to. Can anyone explain what's happening? Many thanks!

It’s sounds as if you don’t have enough available media in the adjacent shots. See if this helps explain how transitions work

www.fcpxbook.com/tips/transitions.html

I've experienced the same issue, It wasn't the available media in adjacent shots.
Just after upgrade to v 10.4 my Preferences get damaged. I had to Reset the Final Cut Pro Preferences. This is how to delete them (Point 11):
support.apple.com/en-us/HT203477
After this, setting Transitions Default time in Preferences worked fine again.