Problem with missing audio on dvd copy

I have successfully backed-up DVD’s before but for some reason this time I am missing audio on half of a movie and then also only on the previews of another movie. I was able to back-up another one which worked perfectly, sound and everything. Is it the CSS? I’ve tried it 3-4 times each with different copying software I.E. Dvdshrink, dvdxcopy…but nothing seems to help with the sound in those certain spots. If anyone could lend some advice it would be greatly appriciated.