Thanks cziehr . . .
I tried that, but unfortunately it doesn't work.
I've narrowed things down a bit since I posted the question.
The 'problem' - or 'feature' - appears to be related to the building of the SearchIndex.
I've rebuilt it a few times (using the SearchIndex Manager plugin), but 'search' still ignores most* instances of plugins codes ( I've found that goes for {{pluginname}} and also pluginname ) .
I've also confirmed that the BatchEdit plugin finds all the entries every time without fail.
I'd like to sort it out, because I'm trying to list all pages which have are using a specific plugin code. I could do it 'by hand' but there are several hundred of them.
*note: Oddly, the search routine does (sometimes) find fragments of the code in some pages, but not the full code.