What version of iDVD are you using?
What system version are you running?
Where is iDVD located and is it on an APFS volume?
Where is the project file located?
How long is the project (from the Project ➙ Project Info menu)?
What type of media is making up the project, i.e. videos, still slideshows?
If there are videos involved have you edited any of the original video after they were added to the project?
Follow this workflow to help assure the best qualty video DVD:
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.
Then burn to disk with Disk Utility at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.
