Small business owner who did a contract job is still waiting to get paid by large corporation who keeps coming up with excuses

I have my own photography business and I do contract jobs for a large corporation. I have never had any problems in the past with getting paid for my work. When I am contacted about a job I give a project estimate, we agree upon it, I begin the project, they send me a PO (purchase order), then I send an invoice, and then I get paid. The last project I did for them began 3 months ago. I sent them an estimate, then part way into the project I had to give them a new estimate due to the extra work the project required (about 5 weeks ago). We agreed upon this. Then I waited to receive the PO. I still haven't received it. I have contacted them numerous times but they always have an excuse (I was sick, out of the country, new admin, etc.) So much time has passed and I still haven't received the PO. - Is this your question? Add additional information