We have a small agency, and have these issues too. Most of the time it is just the occasional slow payed invoice. If they get past two weeks we shoot them an email.

Things we do to lower the risk as much as we can. #1 we get paid upfront for the month. #2 We always ask for a credit card or ACH as part of the contract, we still do get checks because for some that is the preferred method for them, but we try to get the CC / ACH.

For us, if it's becoming an issue over 1-2 months, I'll personally make the ask and find out if something is wrong. I try to give them the benefit of the doubt that the invoice isn't getting to them or was missed.

Something people have suggested is when the payment is late, a nice notification email that just says that you haven't received payment for this month and according to policy, work is suspended until payment is received.

The reason we don't do this is because for us 100% of the time it has been a missed invoice, not an intentional stoppage of payment.