Use MATCH MONITOR, available in sequence mode, to get a good representation of what the export will look like.
Compose text within the preview screen image borders. Text should overlay the background if it's on a the next higher track.
Try applying the LETTERBOX or CROP Aspect Ratio effect to an image or video that has a different ratio than your monitor or export ratio.
If none of this helps, share your project and someone will have a look. It's easy, quick and can be private. Follow these steps...
1. With your project on the timeline, click on MENU at the top-left. Click FILE|BACK UP PROJECT FILES TO FOLDER. Choose a folder and SELECT FOLDER.
2. Locate the saved, numbered folder and upload it to a free server - Google Drive, MS OneDrive, etc.* (Don't upload the export or just the VPJ file.)
3. Get a shared, public link. If using Google Drive click GET SHAREABLE LINK. Click the blue "Change to anyone with the link..." Then click COPY LINK | DONE.
4. Paste that link here, or click the folder, above top-right in this forum, and message it to me. This is private and won't be shared.
* Before uploading, right-click the folder, click PROPERTIES. Look at the File Size to confirm that it's not too big for the free space on the server.