Jim Jackson wrote on 15/10/2020 :
Anyone got any insights into what these devices do? Could it be
damp/water etc affecting the workings?

As you squeeze the trigger, it should feel very springy, then the
tension should suddenly release with a loud click. Piezo generates high
voltage when it is distorted, which basically what happens when struck
suddenly by a tiny hammer - that is the click.

If it doesn't click every time, it is faulty, many are. Damp can short
out the high voltage, but it would still give a sharp click even if it
didn't generate the voltage and spark.