As you can see from the response of the other members it is hard to pin point the problem without further info on your part, what DVDFab product and version are you using ? and the mode you used for the backup ?
also what brand of blank media are you using ? and at what burn speed was used ?
are your blank media the -R or the +R type ? if they are +R was booktype to DVD-Rom used ?
One more Q have you checked the disc to see if it has been closed/finalized ?