Probably that disc is really big. Try to do a two-pass shrinking.
1) Open the disc and first of all remove all unwanted content (foreign languages and subtitles)
2) Set compression level to manual
3) Select a value like 90%
4) Press the backup button, ignoring the error message; i.e save files even if output file dimensions are too big to fit on a single layer disc. REMEMBER: Enable deep analysis to obtain better results.
5) When shrink end backup, close the program.
6) Open again dvd shrink, and load the shrinked movie (not the original).
7) Set compression level as Automatic
8) Press the backup button again
This time dvd shrink should be able to compress corretcly the entire disc.