When I freelanced one of my customers moved the accounting to SAP, which had the unfortunate consequence that they didn't pay me for four month.
It was, however, a reputable, well known firm and I had no reason to believe that they where playing games or trying to bullshit me.
I was right and got payed in full when the glitches got ironed out. Nevertheless, it was a tad annoying and part of the perils of a freelancer, I guess. Thankfully, while being persistent with their accounting department I never blew up and always stayed civil.
Very much second that setting up the invoicing for a new supplier can take time, especially at multinationals or other large firms. I can understand this due to the fact, that procurement is one of the most sensitive areas when it comes to fraud and dodgy invoicing. Thus firms set up a lot of safeguards before actual money flows.