Ah HA! One more of these moments.
As I think it happened many times on that one, I will take 5 minutes and write myself a (google-indexed) note:
For pattern-along-path to work, both objects have to be paths (that’s easy), but more importantly, the pattern one, needs to be a closed path, i.e. a single line won’t work (it’s obvious why: Inkscape can not stretch a single line…).
SO obvious when you know it.