Why won't my external disk drive read some of my CDs?

I recently bought a COOLEAD USB Slim Portable Optical Drive for my new MacBook so I could download my entire CD collection to iTunes. Along the way of downloading my almost 100 CDs, there were 6 random CDs that the drive just refused to read, and it would spit them back out. I tried to get it to read each disc at least 3 times, but it never took them. I thoroughly inspected each disc, and there's not a single scratch on any of them. What could the problem be and how can I fix it to get my CDs downloaded?