DVD+R disks can only be written to once. Once they're burned and finalized, they can't be changed.
If you write data without finalizing, you can add more data, but once they're finalized they act just like DVD-ROM discs.
If you want to erase and overwrite data on DVD, you need to use DVD-RW or DVD+RW discs. Another alternative is DVD-RAM disks, but they have a different format and not all drives can use them.