You sound like you were asking a different question than I thought. You probably have more basis for comparison than I do. It sounds as though you call each of your companies each month and ask when their checks were mailed out. You are comparing the responses you get when you call. Some companies immediately apparently tell you when they mailed checks out and others aren't as open with the information.
I never contact a company to ask them when their checks were mailed out unless payment is at least two weeks later than their stated payment terms. And if a company doesn't pay according to their stated payment terms and I have to call them to see where payment is, I give them one more chance. If I'm forced to call them a second time because they haven't paid, they go on my "don't-for-work" list. I make few phone calls. I figure I earned the money they owe me by doing/reporting the shop. I shouldn't have to spend time chasing my payment. I certainly wouldn't chase my payment when it isn't even overdue.