The 6-month-old invoice aside, we've been running into many more cases since last November of clients "stretching" their invoices -- waiting until the due date

The 6-month-old invoice aside, we've been running into many more cases since last November of clients "stretching" their invoices -- waiting until the due date before mailing it, waiting until we contact them, that sort of thing. Like you say, they're usually quick once we (politely and gently) ask again. I've also been hearing of this from my other clients who are running their own businesses. It seems to be endemic at the moment.