Understanding which tape out of the library will be used next??

I am attaching a screen print of the two magazines that are in my library.  On the left hand magazine,  the slots are number from 1 through 12 .  I do not use the mail slot so I start placing tapes in slot 2.  I had 4 tapes (slot 2, 3, 4 and 5) that were labeled for end of month tapes.  I had two extra tapes in slots 6 an 7 just in case.  Well, the duplicate to tape for all 16 jobs wrote to tapes in slots 2, 3 and 6 and skipped slots 4 and 5.  Why did this happen?  How can I make it use the tapes in order.  They are being appended to and when full, should go to the next tape in line etc.

I know I am asking a lot of questions regarding this tape thing but so far,  you all have answered my questions and I do appreciate that.

Thanks

Cathy