First Consulting alliance do not pay

First Consulting Alliance accepted our quotation to manufacture and install an aluminium door, which clearly states on our invoice 60% Deposit, 20% Manufacture and 20% on day of installation, on day of installation there was a defect on the door (technical adjustment) that needed to be done, which we then sent out a technician to sort out the issue, before they left they tested the door and was in working order, a final invoice was then sent for payment, First consulting then said the was another problem with the door, once again a technician was sent out there were no adjustments to the door, Confirmed with Trace she was happy with the door, final invoice was sent out again, (which was supposed to be paid on day of installation)...Then the door was still not right, arrangements were made with our supplier to inspect the door, all that had to be done was re-align the glass, once again we confirmed is the door working, Trace said yes the door is working , That was the last correspondence we had from first consulting up until Feb 2015, EVERY TIME FINAL INVOICE IS SUBMITTED THERE'S A PROBLEM...IN FUTURE INSIST THAT FULL PAYMENT IS MADE BEFORE ANY WORK IS DONE