Hi Jocelyn,
Not sure why you are seeing that behaviour. As I mentioned, some (many) of the titles can't be modified. Two I've looked at are Lens Flare (as previously mentioned) and Boogie Lights - neither show the "Show Fonts" button. However, others such as Centered and Vertical Drift, do show this button. When the button is clicked a panel opens allowing changes to font colour and size. You can also access the System Font Panel (bottom left of the pop-up window).
From what you are describing, it appears that you can see the Font Panel for some titles but can't click on any of the items listed. Strange!
You may need to trash iMovie's preference file. For iMovie '11 (version 9.0.9) it is named "com.apple.iMovieApp.plist" and is located in the Preferences folder in your Home Library folder. Here is a User Tip by AppleMan1958 which describes the procedure:
https://discussions.apple.com/docs/DOC-4061
Trashing the preference file will not harm your existing Events or Projects. When next opened, iMovie will automatically create a fresh preference file with the default settings. As mentioned in the User Tip, you can change these to your preferred settings in the menu item iMovie > Preferences.
John