oh man, this. So I've been doing contracting forever; usually direct to smaller companies or through a middleman who was also pretty small. And most of the time, smaller companies are pretty good about paying on time, and pretty responsive if you say "Hey, this is really important to me, I've gotta pay the power bill." (my power bill, well, let's just say it could buy my car every month.)
At my first contracting gig using a body shop way larger than I was, well, they'd kinda sort of start thinking about paying me 30 days after they got my net 30 invoice. This /really/ bothered me. In fact I made a lot of unprofessional noise to the client, and ended up staying home, once, until they paid me. The thing was, I was really worried that they weren't going to pay me at all.
It did not occur to me that this was just standard operating practice, and that they were planning on paying me; they just wanted to stretch it out a bit.
They might not even be trying to "stretch" it (a large company could give a crap about the float on an indie consulting invoice); they might just run accounts payable on a state machine in which the part that elapses a timer hasn't started yet. There might be all manner of reasonable reasons for doing it that way; for instance, if they accidentally release payment early, they may screw up projects by blowing expectations.
In any case, yeah: some of your best clients will do a terrible job paying you on time. 'slife in the big leagues.
I once worked in a bank doing tech support and occasionally fixed a few things in the sealed-off internal finance room. The place was littered with red final notices - including utility, phones, everything. As a bank they could easily put to use any extra cash they had, so the official policy was to pay everything late. No doubt everyone knew they would pay eventually, but when you're big you don't have to.
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.