A number of possibilities.
First off, at times, fab can be temperamental with re: to -R media, at least with certain versions and burners. Also, memorex is notoriously unreliable media - very inconsistent. Almost everyone has tried them and moved on.
Try a quality media, like Taiyo Yuden or Verbatim and also give +R a shot...a +rw here would be fine.
Please post some specifics, ie., burner model, media code. You can obtain the media code via tools such as Nero CD-DVD Speed.
(Just click on the link and it will open the latest version as you click download).
Hope this helps.