We have worked for Kaimen for a few years, and they were exceptional about payment--right on time, about 30 day cycle,  and no arguing about price or costs.  Slowly, the time between payments grew.  Then we had to start calling them, and a check would be put in the mail.  Now, we are like you, sitting on a few old, unpaid invoices.   When a good client goes bad, I think we all try to give the benefit of the doubt for a little while, and don't want to alienate a "good" client by hard-line tactics right away.  So, we wait......   and we end up getting stuck.