CDs burn successfully. But then it says they're blank.

I was having trouble burning audio CDs through iTunes. After a minute of burning it would suddenly quit and give me a display message saying it could not burn because of a Medium Write Error. I went online and saw some suggestions to set the burn speed lower. So I set it to 4x. This time the CD burned successfully. Or at least, it appeared as if it did at first. It burned all the songs. And then the CD appeared in my iTunes list as a finished CD, and on my desktop as a finished CD. It was only after I ejected the CD and then popped it back in again that I discovered it wasn't successful after all. When I popped it back in, my computer treated it like a blank CD that had never been burned. Why is this? Why did it seem successful at first, only to prove to be unsuccessful?

Cheap media, some drives can be very snobby about the brands of CDs they will burn to.