None payment

We did work for Sure Assist in April 2017 and to date they have been promising payment every month. When following up with them we get very little communication or no communication for months from them( Ansie and Bianca) we have been emailing and calling for months now. We also received proof of payment that was made to them from the client which was done in May 2017 so we don't understand why are you not paying us for the Job we did???? I cant understand how you can run a business like yours in this manner? We did the work, client was happy so why not pay us?