First tape in set ignored

I set up a duplicate to tape job with 5 tapes in a library. The first tape was ignored, so everything was written to tapes 2-5. Fortunately, I really only needed 4 tapes, but when I use 4 tapes next time, I want to make sure they are all used properly.

One caveat - after I labeled and inventoried all of the tapes, I performed a small overwrite to the first tape as a test, then set all of the real tape jobs to append. The first tape did show up as appendable, so I don't think any overwrite protection was the culprit.

Any ideas? Next time around, I'm going to try setting the first tape job up to overwrite and all the rest to append, but I'm not sure why that would make a difference.