The GUI may say that the tape is at 100% if the default capacity of the device is less that what is being written to the tape. However, NetWorker will continue to use this tape until an i/o error is generated while writing to the tape. This i/o error can occur if:
1) tape reaches its physical end
2) error occurred while writing to that tape
Either way, these two conditions will cause NetWorker to mark the tape as full.
If you are seeing poor tape performance, for example, if NetWorker only writes 10GB before NetWorker marks the tape as full, then try to use something like NTbackup or tar to write directly to the drive and see if you can get better performance without using NetWorker. This will determine whether it is, or is not, a NetWorker issue.