Access to group files when assigned a seat

I have just reassigned a seat to a previous user in Articulate 360 admin. He cannot see the group files he had access to previously. 

He had been unassigned a seat to allow another user to access the Articulate apps. Now I have reassigned him his seat, and he cannot see/access what he used to see/access.

Can someone please explain what I should do to restore his access from a previous seat allocation?