Had another play, and I think I've sussed it
I was part way there with the 'double letters' I mentioned before. It seems to go wrong whenever the first character of the search string happens again anywhere else in the string.
Hence 'example' does not work (repeated 'e') but 'working' does ('w' is not repeated). Other characters in the string can be repeated safely, just not the first one. Couldn't find any other problem strings.
Feel free to use any schematics and algorithms I post on the forum in your own designs - a credit is appreciated (but not a requirement).
Don't stagnate, mutate to create. Without randomness and serendipity the earth would be just another barren rock.