The only thing I can suggest is that you try one of the DVD recovery programs you can find on the internet. Isobuster.com is a well established supplier of recovery software for optical media.
Unfortunately DVD media is not always defect free, and although an immediate verify after write is OK, further down the line, the media just loses the data. Always check the media surface in a good light and reject any disks that have visible defects (not that uncommon in cheap disks)
I would recommend using a good quality (eg WD) external USB hard disk for storing critical images and maybe keep a backup copy on another hard disk. Storage is pretty cheap these days.