Hello Daniel,
Did you run the btape tests? were the tests successful?
All tapes need to be blank tapes before you label them in Bacula.
If, for example, you use tar to write to the tape, then you label this tape in Bacula, only the space not used by the tar command will be available to be used by Bascula.
You may also make sure you don't have volume size limit in your pool definition.
Hope this helps.
Best regards,
Ana