Try updating to current iMovie 10.1.9.
Also, try using a different font, one that has more basic block letters. Graying down a white font can sometimes
help as well, or using some color other than white, red, or yellow. The font might work against a different background, as well.
Another thing you can do is to create a new project, put your title in it, and see if your title works there.
You can check to see if there is a problem with the font itself. Go to Applications/FontApp and open it. From the displayed list select the font that you are using. Do File/Validate to see if there are any problems with the font. If problems are noted, use another font that you have validated. Also, in the FontApp do an Edit/Look for Enabled Duplicates and see if your font is a duplicate font. If so, remove the duplicate. Now see if your font works properly.
There always are workarounds as well. For instance you could create the title outside of iMovie. Take a screen shot of your blank background, open with the Preview app on your Mac (click on the screen shot and it automatically will open in Preview), click on Tools/Annotate/Text, select the font and color you want from the "A" icon in the toolbar, type your text, save the image, and drag it into iMovie.

-- Rich