If it's not an actual rewritable CD (CD-RW), then every time you write more photos to the drive, it is actually burning a whole new selection of stuff to the drive and creating a new "
session." If you can't see the stuff that's on the drive from before, that usually means that it can't find the (multiple) table(s) of contents for those older sessions, or perhaps the total number of sessions is too large for the drive to play nice with. In that case, the data are still on the drive, but the drive might be ignoring them for whatever reason.
The takeaway from this is that you should treat CDs as one-time-use media. Get everything together you want on that CD, burn it once at the slowest speed possible*, and be done with it rather than adding to it repeatedly.
--Patrick
*This improves its chances of lasting a long time.