Like Ben’s response, mine would be similar since I don’t know your level of expertise.
Is this your first time using a NLE program? If so, the underlying concepts of a transition may need to be reviewed.
That said, the obvious question may be: are you first editing the clips in the timeline window (i.e. shortening the ends of the clip)? If you don’t, then there will be no ‘overlapping’ media to create the transition between the current clip and the next clip you drag and drop into the timeline.
For instance, a dissolve between two video clips is an effect of two ovelapping clips that are transitioning between each other. The students I teach often have a hard time grasping this concept when they first take on editing. So please excuse me if that was overly obvious to you (not sure your level of expertise on editing).
If your problem still persists, there may be system-based causes; I might suggest the following steps to try:
1. Repair Disk Permissions. After quitting FCP, going to Utilities/Disk Utility and performing a ‘repair permissions’ operation on your system disk.
2. Check for software updates, especially for final cut pro and pro applications (there may be a recent update that you have not run?...
3. Make sure QuickTime is updated to the current version (software update should fix this)
4. Try using other media files and a new project file, or open a previously created project (maybe even one created on a different computer) to verify the problem still exists on the system and is not in the project file (i.e. a corrupt project file)
and if none of those work, then consider:
5. Re-install FCP from the installer, and update as described above.
Let us know if this helps at all…