Student visa asking for documents I can’t obtain?

I was accepted into a grad program at a University in Japan and they want CoE documents sent to them by early December for April semester start date.
I sent them my filled in forms to check, and when they saw I had attended a language school (~10 years ago) and a study abroad in Japan, they asked for a few documents from each.
I've tried to contact both schools to obtain the documents. However, I haven't been able to get anything from the language school since I left them – I've tried before. And the other Uni says they can only provide two of the three documents my school says are required.
Does anyone know what I can do if I can't obtain these? When I try to ask my school they say "we understand, but we need the documents" – the usual Japanese paperwork stuff. I was able to get a visa twice before without this issue though so I really hope it's possible.

by realgamushara