Memorex (and most brands) do not make their own media. They often change manufacturers without any hint on the packaging. Sometimes you can tell by where it is made, but usually you need a tool to read the Media ID on the disc itselft (such as Nero Info Tool).
Most likely this is what happened. The new discs do not play well with your recorder.
Perhaps you could check one of the discs that do work on a computer and see who makes it. It may be possible to track down other brands that use the same manufacturer. Or you could order some online. Many places have sample packs for you to test with.
I normally deal with American Digital and Rima.com.
