Help in determining an error condition

[matiz]

I have an application which receives documents from different people. Some people send the documents daily however there are some for whom I would never be sure of the frequency. I collect the information about number of documents sent by each person everyday. There was a problem which happened recently. One person thought he was sending the documents, but I was not receiving the documents. The problem is that we came to know about the problem after 2 months.I want to be able to determine based on the past / current pattern of receiving the documents that there might be an issue with some one. For example if a user is sending me documents everyday and then I do not receive the documents for 5 consecutive days, this indicates a problem. However if someone else is sending me documents twice in a week, not receiving documents from him for a week might not be a real problem. What would be the best way of doing this?

Thanks in Advance for your help