Clients send their documents however suits them: by email, in Viber, on paper at the end of the month. Someone on your team gathers them from several places, checks what is missing and calls. Then calls again.
The client, for their part, does not know whether the documents arrived and whether everything is in order, so they call too. Deadlines do not wait, and the time goes into collecting paperwork instead of accounting.
This is not a staff problem. What is missing is one place where the client uploads documents and sees the status, and the accountant sees what is missing. That is what we build.