You don’t say if you are using an FCP or third-party title. I know some third-party titles only extend so far. The same may be true of certain FCP titles. It may be a function of the resolution of the FCP project and the resolution of the title not being the same.
An easy way to accomplish what you want is to create the title in Motion and Publish it for use in FCP. When you open Motion and select Title, Motion will open with a Group containing a drop zone layer, and a title layer. I’d delete the drop zone (usually used for a full screen background image) and replace it with the subtitle background you want. Then, using the Inspector, adjust the title layer to the position, size and style you want. You can then Publish all those parameters, so you can change them in FCP. There are a lot of tutorials online that explain the details, if you’re unfamiliar with them.