NG&G Unpaid Invoice Lincoln Rhode Island

We have had numerous requests to work for NG&G to fix and repair retail stores but until they pay the outstanding invoice from summer of last year, we will not help them at all. I keep getting calls to do more "emergency"work and I always ask about the unpaid invoice but they never seem to get the check in the mail. They even sent emails requesting any unpaid invoices be sent in again (3rd time we sent in invoice) and they would be paid before the end of 2017. They did not pay the invoice and today I got a call and told them to look and pay but the lady who called acted like she was concerned for our payment and said if she was in charge of my account, I would have been paid already. She then hung up. Go figure!