Property managers are notorious for late payments, I think the invoice has to pass through too many hands before it gets to the person who issues the payment. You can try to find that persons email and send them the invoice directly. I have had success with emailing or calling that person and explaining to them what the terms of payment are on your end and see if that works for them or they need more time. Better late than never.
Also, the reality is a customer who consistently doesn't pay on time is a waste of time. Chasing payments takes up your time and no one charges for that. I easily drop clients who I have to chase month after month, it is just not worth it.
