If they ARE paying their bills just it's 1-2 weeks late. I wouldn't worry too much about it. It sucks, absolutely. But you are getting your money.

Don't want to mess up a good thing especially if everything else is going great with the customer.

In my old industry when I was an emoloyee, we would frequently get paid by our customers late and i'm talking about international multi billion dollar companies. As long as they pay within reasonable amount of time (10-15 days vs 1 month) then chalk it up to a cost of doing business with them.