Again, if the tapes are full, no matter what the settings, they will not be appendable.
Do you have all your jobs defined as "APPEND"? Normally, you have an Overwrite job that starts the tape, the Append jobs that follow and write to the same tape. If all your jobs are Append, try changing them to "Append, else Overwrite" This way, if an appendable tape is mounted, they will append.
if they can't find an appendable tape they will try to overwrite. If you have a 5 day OPP and 15 days, this should work.
If this is still clear as mud, let me know