That's one of those "ebb and flow" factors that inexperienced business owners get out of a book.
They tell you that the best way to manage your cash flow is to get your money as soon as you can and then delay paying anyone for as long as you can. But what they don't tell you is that it can mess up a perfectly good relationship with a vendor.
I had a similar situation where they didn't send my invoice to the accounts payable dept until after 90 days but when I wanted to retain their services, they told me, "We need the full payment in advanced."
That was the last time I would ever do any work for them again.