Dishonest and disrespectful!

I am absolutely appalled and disappointed by the treatment and service I received from Ogilvy and Mather, Bryanston. I rendered a service to them, In October 2017, and to date (01 February 2018), I have not received my payment for the services I had rendered to them when they were doing a campaign for multichoice. I handed in my invoice(which they hounded me for), to one of the employees. On the day I rendered the service, she told me that she was leaving the company and would duly transfer my account to another person who would be filling in her position. I was in communication with the person, Gail Mukhonasi who requested all my documents to effect payment, and I handed those in timeously. I was informed that payment would take 30 days and was sent a confirmation of the purchase order, which indicated that the payment had been processed. In the new year(after having not received any payment), I contacted the said person, who denied me ever having sent any invoice and asked me to resend it, which I did. to date, there each time I request her to let me know when I will receive the payment, the answer is 'soon, or payment will be released this afternoon. It has been three months since the service has been rendered and I am utterly frustrated at the gross lack of professionalism and disrespect in which they treat their suppliers! After having worked on their project, diligently and thoroughly, it infuriates me to have to be given the runaround when it comes to getting the money paid to me. I find their dealings very exploitative and unprofessional.