I would just advise to make sure something internal isn't holding up payment at the customer. If this is a larger company, any number of things could be causing

I would just advise to make sure something internal isn't holding up payment at the customer. If this is a larger company, any number of things could be causing it. If it is a small company or an individual, then disorganization could be a cause...or a lack of funds. My companies have faced this issue in the past with companies from the Fortune 50 to small companies. Very few times did something actually go unpaid for longer than 90 days. Just be friendly and persistent.