Ritek media are not exactly the very best, so if you can find them I suggest to get some Verbatim discs.
The main problem of Ritek media is that they deteriorate very fast in time.
I suggest to check your oldest burned discs to see if they are still readable. If you notice some difficult but the disc is still readable, I suggest also to recover all data immediately and burn again on a better quality disc.
To recover data from a disc the easiest way is again imgBurn. Select the Read mode, then save an ISO file on hard disk. After burning the ISO file in Write mode (wait a couple of minutes to cool up the burner and the disc), in ImgBurn select the Verify mode and then check the just burned disc to see if there are errors.