What version of Premiere Elements, what computer operating system?
What is your burn to DVD - burn to DVD-VIDEO standard or widescreen on DVD disc or AVCHD on DVD disc?
What are the properties of the Timeline files, and what is selected as the project preset to match the properties of the source files?
What does the burn dialog show for Space Required and Bitrate (with check mark next to Fit Content to Available Space?
Does the problem exist with or without Timeline markers and disc menu?
Depending on what version of Premiere Elements that you have, can you burn to ISO Image or Folder if you cannot burn to DVD disc from within the project?
Does the problem exist even if you start a new project?