Former property manager sent security deposit to wrong address even though they acknowledged the receipt of forwarding address

Hello. I noticed that in my USPS informed delivery that the former PM sent a certified letter to the property we moved from even though we filled out a notice to vacate property form with our new address a month before the lease was up. I have the email stating they received the form. I notified them of their mistake and asked if it will be forwarded by USPS since it is a certified letter. The 30 days that they have to return our deposit is up in a week. The property manager told me that she’d have to look at our file to double check the form and get right back to me. I tried to follow up the next day and was told that she’s been busy and she never got back to me. What is the next step? Tracking shows that the letter was undeliverable due to the address and there hasn’t been an update.