This assumes that you have actually changed your project to Bluray.
In the project panel, look at the colum "DVD Transcode Status." This should be "Untranscoded" until you actual transcode to DVD. If it is not "untranscoded," right click the asset for that timeline, and pick "Revert to Original."
Look at the column "DVD Transcode Setting." This is probably a specific setting that is too big for the DVD. First try changing it to au tomatic. If that does not work (and it may not going from HD to SD inside Encore), you will have to apply a specific preset with a calculated bit rate that will fit the DVD.
If that doesn't make sense, post a screenshot of the project panel where we can see DVD and BD settings.
Edited to put in omitted words!