The only type of optical disk that you can erase are CD-rw or DVD-rw - well, and RAM-DVDs, if you have such.
Anything else is usable to record or burn only once, So if you erase, it's done, you can't reuse it.
Which do you have?
If you are using Re-writeable disks, then you can try cleaning the drive, or try a different brand of disk.
In my experience, re-writeable disks are quite limited in the number of times they can be erased.