I've worked in financial service since I was in college. In fact, my very first job was as a collections agent for a big, evil bank. For four hours a day, five days a week, I'd sit in a call center in suburban Southern California with a fugly headset on, connected to an automatic dialer. I would speak to people who were past due (bank speak for late on making a payment) on their auto loans. The majority of the time the person on the other end of the line just spaced and forgot to make their payment.
The bank had entire department dedicated to closing the gap between revenue that was owed to them (AKA receivables) and cash. The gap is entirely created because of how, when or if a customer will pay their bill or invoice. As a small business owner, you probably experience the drawbacks of this gap like not being able to pay your vendors or yourself or your other bills. But there are things you can do to help minimize the gap. This is called managing your cash inflow.