multiple sessions burned on a CD/RW won't read

I have an iMac running OSX.3.9 and use an external Iomega CD/RW drive with Discribe OSX 5.1.6. I have several CDs with multiple sessions burned following the instructions to "open next session" each time. If I load the CD into the burner and check the disc info, it shows that the sessions were burned but when I insert the CD in my slot loading iMac, only the first session shows. Any idea what I can do to retrieve the rest of the data?