I've been doing multiple month-long contracts for a client as a software engineer, and the CEO is regularly late with payments despite me raising this concern. It's gotten to the point where I don't know what to do further about it and I am considering just stopping doing work with them after my current contract is done. I'm not sure what I can say even further without being pushy, since I've sent several reminders for various invoices. They do get paid, just late and it's a distraction to have to deal with them being late.
They recently closed a round of funding so it's unlikely a cash flow issue.