Opmed! It's not alive!

I treat only a few patients from Opmed, but they are slow to pay and do not respond to emails when requesting payment or other requests. I requested PMB treatment for a lady last year October, she has been into their offices 4 times and still, they do not respond. I email them and they do not respond, I know they are getting their emails because when I request a statement from them using the same email I get a response although weeks after the request. I am still waiting for payment from a treatment done 4 months ago. I have contacted them several times to request payment. I have notified the member and she has been in and still, they have not paid.