I usually agree terms on accepting dates. Many stick to it and are very good.
I was once stung by a company locally that I thought had good business relations with, but they kept refusing to pay. “Payment was sent xxx” and it hadn’t been, “We’ll clear it Tuesday” and it wasn’t, only to be replaced by “we’ll clear it xyz”. Took over 3 months of chasing and had to threaten legal avenues before it was paid. Turns out it is a company-wide problem and have been approached by other locums telling me not work there since (I won’t be going back).
How long have you been waiting for them to pay?