Plugboard "Metadata: Show series [series index] - title as title (Kindle)"

I'm having trouble automatically having the kindle show the series name and the book's place within the series in the title. For example, The Man Who Went up in Smoke is the second book in the Martin Beck series of police procedurals, and I'd like the title on the kindle to be (or to start with) Martin Beck [2].

I copied pasted the following from the sticky thread on techniques to use plugboards, etc.:

format: mobi
device: kindle2
template: {series:|| }{series_index:0>2s|[|] - }{title}
destination: title

Because of something I'd read on another thread, which I can't find again at the moment, I created a second plugboard that was identical except that the format was PRC.

The result was a bit surprising. It worked perfectly for some books and not for others. I could not see a pattern to what worked and what didn't. All of the books have the correct information in calibre's "Series" column. Sometimes, one book in a series worked and another didn't. Series with many books worked; a series where I have only one of the books (so far) worked. I'm having trouble thinking of anything that distinguishes the books for which it worked from those for which it didn't.

Any thoughts?:help: