Post Indexer misses one tag on a post?

In general, Post Indexer is working fine on our multisite; see here for tag cloud so far. But one student emailed me that her post is not found via our course tag, "envsintro," and she's correct, though it can be found via her other tags (some misspelled) including this one, this one, and this one. Do you have a possible diagnosis? I've checked, and she does have the correct course tag in addition to those above.

Many thanks,

Jim P.