we had this with dd1 when she was in ms, and when she was in her (hopeless, for her) ASD school.
dd1 has always been very chatty, but when she was younger she was very hard ot understand - this was written off as 'babble' by staff in ms, when it was clear as a bell to me (although, to be fair, I watched some home video the other week, and couldn't understand half of it
)
her other trick was to say everything really, really, quietly, with minimal facial expression/movement, and so a lot of her communication wa smissed. I remember standing by the door, and knowing what she was saying from across the room (she was asking for a drink, at the time when we were working really hard on getting her drinking again after her mammoth strike), and nobody noticed. that made me wonder what else was being missed.
I think this kind of thing can become 'just' part of the culture sometimes - all parcelled in with expectations in a way. ie the staff know your ds is minimally verbal (NOT the same thing as 'having no functional language'), and so they don't really expect him to be asking for something, or commentating on anything, or communicating ina meaningful way, and so a lot of his efforts go overlooked.
dd1's current school is excellent at noticing it all, and her functional language has increased massively a a result (and it's not as though she wasn't a chatty little thing anyway) - no attempt at communication goes un-noticed, as far as I can tell (not much access to the classroom, but I do see her in theplayground, and she runs herself ragged at hometime saying goodbye to everyone - I have no idea of who some of the tutors are as they are from the secondary unit, but she trots off to speak to them all and they all certainly know her and her language foibles).
nothing is ignored or overlooked - it may be shaped, if inappropriate (ie interrupting a conversation - one of dd1's major targets at the moment is waiting to be answered, or judging when and how to enter a conversation), but every response is noted, and collated, and pondered over (if necessary), even if it is not immediately responded to (for whatever reason)