I don't use DVD clone, but perhaps the clips you're trying to backup/copy aren't DVD compliant.That would be my first guess, since you've provided no specific details..
Are the DVD's compliant, IOW, do they contain a VIDEO_TS folder with VOB's, BUP's, and IFO's..
One quick reliable method to find out, would be to open the disc in DVDshrink and analyze..
I fit spits out an error, chances are it is not compliant..
Any protection on these DVD's?