/jp/ - Otaku Culture

I've been thinking a lot about the comprehensible input hypothesis lately and I suddenly had this moment of clarity.

I know a lot of people, typically the children of migrants, who can understand their mother tongue 100% but who cannot speak it competently at all. (In some cases they can't even formulate a grammatical sentence despite having more or less perfect oral comprehension.)

How do you reconcile this common phenomenon with the ideas behind comprehensible input? If the major ingredient in language acquisition is understanding input, with the idea that input that si understood translates itself into productive competence, how do you explain these cases?