Some one want to explain to me why the accounts re

Some one want to explain to me why the accounts receivable and revenues don't match up? I believe when you book a receivable there needs to be an associated revenue with the receivable in this period or in prior periods. So where I'm confused is there is 81k in receivables. However, that 81k in receivables should result in over 81k in revenue for this period plus prior periods. Doesn't add up. 19k revs this quarter plus 10k last quarter - doesn't add up to over 81k.