Late commission payment

I worked as an agent for a large well known company. My contract was terminated after a 3 month notice period and I was given compensation. I submitted an invoice for the last month I worked and this was agreed and sent to the outside agency for payment.
The invoice has now gone 2 weeks over the agreed contractual payment in 30 days.
I am having no luck obtaining a payment date from either the outside agency or the company them selves. I have sent 2 emails to the outside agency without a response and 3 emails to the company where they have replied they are liasing with the agency for a payment date but a week has gone by without them coming back to me with any date .
What do I do next ?