Why do predatory mites have to be introduced multiple times?
You are right that ideally it should be enough to apply them once, but predatory mites seem to be quite sensitive: If it gets too cold or too hot or the moisture is too low, they might die.
I applied several rounds of predatory mites in non-optimal conditions without any success. They just kept disappearing without any effect whatsoever. Funnily, some small type of Heteroptera came to save my plants. (from nature without my doing)
Next time I would try some Chryson instead.